Worldwide, acute respiratory infections are the most common illnesses in people of all ages. Winter months see increased viral respiratory infections, which are caused by many different viruses. The common cold, influenza and other respiratory infections may occur concurrently in the community and as the symptoms are often similar, it is often difficult to assess the relative impact of each respiratory virus during an outbreak situation. Viruses cause about 80% of upper respiratory tract infections. Most recently we have seen the emergence of SARS (Severe Acute Respiratory Syndrome).

 
How do I recognise a viral respiratory infection?
Common cold
It is estimated that adults of all ages suffer 2-3 colds per year and pre-school children have an average of 6-12 colds per year1. The common cold, a viral infection of the upper respiratory tract, can affect all age groups and can be caused by any of up to 200 different viruses. Rhinoviruses cause up to 40% of common colds. Coronaviruses are responsible for up to one-third of common colds.2 Other causative viruses include parainfluenza virus, respiratory syncytial virus and adenovirus.

Rhinoviruses are responsible for cases of the common cold in the general community as well as in institutional settings such as schools, day care centres and hospitals. Rhinoviruses and coronaviruses have been found to cause a greater disease burden in elderly people living at home, compared to influenza virus or respiratory syncytial virus.3 Rhinoviruses cause infections all year round, with one peak in the autumn, usually as a result of children returning to school.

Colds tend to begin slowly, with the first symptom usually a sore throat, followed by sneezing, a runny nose and nasal congestion. Children may also develop a slight fever (raised temperature). The symptoms usually last for around seven days, but may last longer in some people. Viral shedding in nasal secretions can continue for up to 3 weeks.4

SARS
A previously unrecognised strain of coronavirus has been detected in a high proportion of SARS patients. It is still uncertain that it is the cause of SARS, so the agent is referred to a ‘SARS associated corona virus’. Although the new coronavirus is still the leading candidate, there may be other micro-organisms involved in causing SARS.

SARS generally begins with a fever greater than 100.4°F [>38.0°C]. Other symptoms may include headache, an overall feeling of discomfort and body aches. After 2-7 days, patients may have a dry cough and have trouble breathing. The incubation period for SARS is typically 2 to 7 days, but may be as long as 10 days.
Influenza
Influenza, or ‘flu’, affects all age groups, with outbreaks tending to occur in the winter and early spring. There are three types of influenza viruses: A, B and C. Type A constantly changes with new strains appearing regularly and is usually responsible for the large epidemics. Influenza A is usually a more severe infection than influenza B, which causes smaller, more localised outbreaks. Type C is less common.

Influenza virus can be shed before symptoms appear and up to 7 days after onset of illness, therefore people are potentially infectious before symptoms develop as well as after symptoms appear. The young are at risk because they have not usually developed immunity to the virus. The elderly and persons with underlying health problems are at increased risk from complications.

Common symptoms of the flu include sudden onset of fever, headache, chills, fatigue, muscle aches and pains, runny nose, sore throat and dry cough. The symptoms quickly become more severe than those of a common cold.

Other viral infections
A variety of other respiratory viruses can cause ‘flu-like’ symptoms, sometimes with infection of the lower respiratory tract. Respiratory Syncytial Virus (RSV) can infect the same person several times during a lifetime. It causes more severe illnesses (e.g. bronchiolitis, pneumonia) in children, but only a ‘common cold-like infections’ in adults. It can also produce a flu-like illness indistinguishable from influenza. RSV affects about 90% of children by the age of 2 years5. It is often carried home by school children and passed onto their siblings within the home. The virus can spread rapidly in day-care centres, hospitals and nursing homes. Infections occur mainly in winter to early spring and are associated with high incidence of secondary pneumonia and death in the elderly6.

Human metapneumovirus (hMPV) is a closely related to RSV. It was only recently identified in 2001. It is associated with mild respiratory infections as well as severe bronchiolitis and pneumonia. hMPV infections are thought to occur mostly during winter. The number of people that suffer from hMPV each year is still to be determined. Infection occurs in infants and young children but hMPV has been found in older children and adults suggesting re-infection may occur later on in life.

Parainfluenza viruses (PIV) are a major cause of acute respiratory tract infections. They may cause lower respiratory illnesses (bronchitis, pneumonia) in young children. In older children and adults parainfluenza virus causes upper respiratory illnesses (e.g. common colds) which are usually only mild. Parainfluenza peaks in the late autumn to early winter.

Most lower respiratory tract infections due to adenoviruses are mild and indistinguishable from other viral respiratory infections. Adenoviruses are a less frequent cause of LRTI in children than RSV or PIV, but can cause epidemics of severe LRTI in young children. Adenoviruses are implicated in 5-11% of upper respiratory tract infections, and are also implicated in cases of pharyngitis, pneumonia, bronchiolitis and croup in children.7
How are cold viruses spread?
The routes of transmission of common colds are still under debate. The commonly held belief is that colds are spread by particles of infected mucus generated by coughs and sneezes. When someone is infected they can shed millions of virus particles in the mucus they produce. However, increasingly there is evidence that infection occurs not only by inhalation of mucous droplets but also e.g via the hands. This can occur when fingers become contaminated by contact with the infected nose, or when surfaces such as handkerchiefs and tissues, tap and door handles or telephones become contaminated by droplets of infected mucous shed from the nose8. The virus is passed onto another person either by handshaking or when contaminated surfaces are touched by that person. It is not known which transmission routes are the most important.

Common cold viruses infect the nose lining and are found in very high levels in the mucus. Coughs and sneezes produce an aerosol containing virus-laden mucus particles. The smaller particles may remain suspended in the air for hours and persist for some time. Although it is believed that someone who is coughing and sneezing spreads the common cold very easily, it is very difficult to demonstrate this means of transmission in the laboratory. Some experiments have tried to demonstrate that colds can be spread via aerosol transmission9,10. Volunteers with colds played cards with healthy volunteers for twelve hours whilst prevented from touching their nose or eyes by means of a large neck collar and arm brace. Just over half the healthy volunteers developed colds and the experimenters concluded that infection could only have occurred via aerosol transmission.

The hypothesis that colds are spread via large particles of mucus which settle rapidly onto surfaces is consistent with evidence suggesting that colds are not particularly contagious. In this situation infection occurs only at close range when someone is sprayed with droplets of mucus that enter the eye or are inhaled via the nose.

Although there is evidence for spread of common colds the airborne route, there is little evidence that coughs and sneezes actually produce an aerosol of infected nasal mucus. In one study, volunteers with colds were housed in a room and the air sampled for virus; although 82% of the air sampled no virus was detected.11 When volunteers were asked to cough or sneeze directly onto a surface designed for virus detection, virus was recovered from only 2 in 25 volunteers.

Coughs and sneezes tend to spray saliva from the pool at the front of the mouth rather than droplets of mucus from the nose. Saliva contains little or no cold virus and thus aerosolised saliva is unlikely to spread infection. Colds are not caught by kissing as cold viruses do not infect the mouth and saliva contains very little virus. When volunteers infected with common cold virus kissed ‘cold-free’ volunteers for up to 1.5 minutes, only one case of cross infection occurred in 16 trials.9

By contrast infected nasal mucus readily contaminates the hands when they are used to wipe the nose or block a cough or sneeze. Infected mucus may then contaminate commonly touched surfaces such as desks and door handles. The chain of infection is completed when an uninfected person touches the mucus-contaminated surface and contaminates their hands. They then infect themselves by touching their own nose or eye. The eye acts as an entrance for infection as virus enters the tear fluid, which drains down a duct into the nose. In a study where asthmatic children were trained not to touch their nose and eyes so frequently12, a reduction in self-inoculatory behaviour was observed which was associated with 47% reduction in laboratory diagnosed cold infections and 45% reduction in cold-associated asthma attacks.

Indications are that cold viruses deposited on surfaces can remain viable in large numbers, for several hours.13,14,15. By contrast with bacteria the ‘infectious dose’ i.e. the number of viral particles required to cause infection may be very small.16, 17 For rhinovirus the infective dose may be less than ten.9 It has been shown that infectious virus can be recovered from naturally contaminated objects in the surroundings of persons with rhinovirus colds and that clean hands can readily pick up the virus by touching or handling such objects.18,19

Rhinoviruses can survive for several hours on the hands, and self-inoculation by rubbing of the nose or eye with virus-contaminated hands can lead to infection in susceptible hosts.20 After handling contaminated coffee cup handles, 50% of subjects developed an infection.12 The importance of contaminated fingers in spreading the common cold was demonstrated amongst families. Mothers who regularly disinfected their hands with a dilute iodine solution had a slightly lower rate of infection than mothers using an inactive hand wash.21

What about the SARS virus?
Based on current evidence, close contact with an infected person poses the highest risk of cross-infection from one person to another. Infectious mucous droplets are produced by sneezing, coughing etc. These droplets may contaminate the hands or can settle on nearby objects or surfaces. It is thought the virus may remain infective for up to 24 hours on dry surfaces. Other coronaviruses studied to date have not remained viable beyond 3-4 hours.15 As with the common cold, it is thought that the virus can be spread by inhalation of infected droplets, or by people touching other people, or objects and surfaces that are contaminated with infectious droplets. Infection then occurs by transferring the virus from the hands to the eye(s), nose, or mouth. It is also possible that SARS is spread more broadly through the air or by other ways that are currently not known. SARS, like colds, appears to be less infectious than influenza.

Indications are that spread from faecal matter infected with the virus was the cause of the majority of the 300+ SARS cases in the apartment block outbreak in Hong Kong. An official investigation concluded that leaking sewage pipes and inadequate seals on U-bends were major contributors to the outbreak and that airborne particles carried the virus throughout the complex.
What about influenza?
Studies have shown that influenza virus is also shed in large numbers from an infected person. Influenza can be spread from person to person by aerosol transmission due to sneezing and coughing. Like colds it can also be spread via the hands by contact with objects that an infected person has contaminated with infectious nose and throat secretions although there is less supporting evidence for this mode of spread than for colds. Influenza viruses can survive on surfaces such as stainless steel and plastic for 24-48 hours22 and for up to 12 hours on soft surfaces such as cloth, paper and tissues. Influenza A virus was transferred from contaminated stainless steel surfaces to hands for 24 hours after a surface was inoculated. It easily spreads in institutional settings and crowded places, for example, an outbreak of influenza virus in a nursing home may have been mediated by staff either via contaminated hands or fomites.23
Other Respiratory Viruses
Human Parainfluenza Virus (HPIV) and Respiratory Syncytial Virus (RSV) may survive sufficiently long enough in the environment to allow transfer of infectious virus to hands that contact contaminated surfaces.19,24 The transfer of HPIV from stainless steel discs to clean fingers supports a role for surfaces in the viral contamination of hands.19 HPIV was recovered from hard surfaces up to 10 hours later when the surface remained moist. The virus persisted on hands for a minimum of 1 hour and on dry surfaces for up to 2 hours.25 Parainfluenza virus may have an infective dose via the nasal route of less than 80 particles.16 

Transmission of respiratory adenoviruses is by aerosolized droplets reaching the conjunctiva, nose or throat or by the faecal-oral route. Close contact appears to be necessary for infection to spread from one person to another, and thus illness spreads rapidly in closed environments.7

If I get a cold or the flu, will it help build my immunity to further colds or flu?
Spread of common cold infections is dependent on viruses circulating around the community and a supply of susceptible noses to infect. In isolated communities that do not have regular contact with the rest of the world the community can be free of colds until visitors introduce new viruses. There are many reports of epidemics of colds occurring in isolated island communities after the landing of a ship that brought in visitors with colds but the colds die out as the population develops resistance.

Unfortunately, one of the main problems with cold and flu viruses is the way in which new strains are continuously developing which means that immunity built against one strain will not protect us against the next one that appears.

Practical advice to prevent the risk of infection within the home
There has been a tendency to assume that, since nothing can be done to cure colds or flu, that catching these viruses is inevitable. We can seek vaccination that provides effective protection against the most likely strains of flu, but otherwise, colds and flu advice tends to be focused on how to cope once infected.

Avoidance of cold and flu viruses is preferred, but trying to prevent the spread of such infections through good hygiene is particularly worthwhile in families where there are people more vulnerable to infection, something that is becoming more and more common. For these people the consequences of even of a cold may potentially be much more serious.

Adopting basic hygiene measures will reduce the chances of getting and spreading infection in the home, particularly when someone in the family is already infected. Where there is a significant risk of spread of SARS in the home appropriate hygiene measures should be rigorously adopted.
 
ADVICE FOR CARERS AND THE FAMILY IF YOU ARE ALREADY INFECTED
Treat your hands as potentially contaminated and avoid touching your nose and eyes. Avoid touching your nose as much as possible. Block coughs or sneezes with a tissue or with your hands - but remember they are now contaminated and could spread infection.
If someone leaves a tissue lying around on surfaces be aware that your hands will be contaminated after touching it – so wash them immediately whenever possible. Use disposable tissues rather than a cotton handkerchief to blow your nose. Dispose of tissues immediately and ‘safely’.
Wash your hands thoroughly using soap and water after touching any suspect item. Rinse them under running water to remove any infected mucus. Make sure your hands are dry, as wet or moist hands are more likely to spread germs. Always wash your hands thoroughly using soap and water. Make sure you use good mechanical action and rinse your hands under clean running water to remove any infected mucus. Make sure your hands are dry, as wet or moist hands are more likely to spread germs.
Alternatively if a washbasin is not available a wet-wipe tissue or alcohol-based gel can be used to clean the hands. Alternatively if a washbasin is not available, a wet wipe tissue or alcohol-based hand gel can be used to clean the hands.
When looking after a baby with a continually runny nose, keep a plastic bag with you to collect tissues until you have the opportunity to dispose of them.  
Be aware that you and other family members could pick up infection from hand contact surfaces around the home. Ensure that surfaces frequently touched by different people are regularly cleaned and disinfected, e.g. door and cupboard handles, toilet flush handles, wash basin taps and telephones. Use a bleach-based cleaner that is capable of killing viruses*. Think before you shake hands with anyone. For disposal of faecally contaminated material, using gloved hands, remove as much as possible of the vomit, excreta, etc., from the surface using paper or a disposable cloth. Clean the surface using detergent and warm water and then apply a disinfectant. Use a bleach-based cleaner that is capable of killing viruses.
Clean and disinfect kitchen surfaces before preparing food on them using a bleach-based cleaner*. Clean and disinfect kitchen surfaces before preparing food on them using a bleach-based cleaner*.
Cleaning cloths and sponges can readily spread germs from one surface to another. Make sure that cloths are disinfected immediately after use using a bleach disinfectant* and thoroughly dried until next use. Alternatively use a disposable cloth or wipe to clean surfaces. Cleaning cloths and sponges can readily spread germs from one surface to another. Make sure that cloths are disinfected immediately after use using a bleach disinfectant* and thoroughly dried until next use. Alternatively use a disposable cloth or wipe to clean surfaces.
Do not share bed linen, towels, facecloths, toothbrushes, eating utensils etc with an infected person. Do not share bed linen, towels, facecloths, toothbrushes, eating utensils, etc.
Wash any laundry (especially handkerchiefs, towels, face-cloths) used by ill people separately from other laundry, and at a higher temperature (at least 60ºC) to ensure viruses are inactivated. For clothing which is soiled with faecal material, handle using gloved hands and launder at 60ºC wash using a bleach-based product.
Make sure that as far as possible the home is kept well ventilated, and avoid spending time in rooms which are poorly ventilated. If you are infected with the SARS virus, stay indoors and keep contact with other people to a minimum until seven days after your symptoms have improved and you have had no fever for at least 48 hours.
If someone in the home is infected with SARS wearing a mask will reduce the spread of infection by preventing you from touching your mouth and nose – but remember to also avoid rubbing your eyes. If you are infected with the SARS virus, wearing a face-mask to prevent the spread of infectious droplets.
   * Bleach disinfectant should be diluted to contain at least 1000 ppm available chlorine. Typically domestic bleaches contain 5% w/v (50,000) ppm. Note: the chlorine concentration of bleach products will decline if products are stored at room temperature for prolonged periods of time. In Germany, the use of aldehyde-based disinfectants is also recommended as an alternative to bleach.

You should also be aware that:
· Although mucus in the nose and throat can be a source of infection for others, the production of mucus helps the patient as it traps bacteria and viruses and limits their spread in the body.
· Hot tasty drinks and spicy foods increase mucus production and this can help relieve symptoms of sore throat and cough. In contrast, a hot dry atmosphere in a room can dry up mucus in the nose and throat and make us more vulnerable to infection.

 
References
 
  1. Turner RB. Epidemiology, pathogenesis and treatment of the common cold. Ann Allergy Asthma Immunol 1997;78:531-539.
  2. Myint SH. Human coronavirus infections. In Sidell SC, ed. The Coronaviridae. New York: Plenum Press, 1995, p389-401.
  3. Nicholson KG, Kent J, Hammersley V, Cancio E. Acute viral infections of upper respiratory tract in elderly people living in the community: comparative, prospective, populations based study of disease burden. BMJ 1997;315:1060-1064. 
  4. D' Alessio DJ, Peterson JA, Dick CR, Dick EC. Transmission of experimental rhinovirus colds in volunteer married couples. J Infect Dis 1976;133:28-36.
  5. Simoes EAF. Respiratory syncytial virus. Lancet 1999;354:847-852.
  6. Nicholson KG. Impact of influenza and respiratory syncytial virus on mortality in England and Wales from January, 1975 to December, 1990. Epidemiol Infect 1996;116:51-63.
  7. Cherry JD. Adenoviruses. In Textbook of Pediatric Infectious Diseases Vol 2. Ed Feigin RD and Cherry JD. pp1666-84. Philadelphia: WB Saunders, 1998.
  8. Goldmann D. Transmission of viral respiratory infections in the home. Paediatr Infect Dis J 2000;19:S97-S102.
  9. D'Alessio DJ, Meschievitz CK, Peterson JA, Dick CR, Dick EC. Short-duration exposure and the transmission of rhinoviral colds. J Infect Dis 1984;150:189-194.
  10. Dick EC, Jennings LC, Mink KA, Wartgow CD, Inhorn SL. Aerosol transmission of rhinovirus colds. J Infect Dis 1987;156:442-448.
  11. Hendley JO and Gwaltney JM, Jr. Mechanisms of transmission of rhinovirus infections, Epidemiol Rev 1988;10:242-258.
  12. Corley DL, Gevirtz R, Nideffer R, Cummins L. Prevention of postinfectious asthma in children by reducing self-inoculatory behaviour. J Pediatr Psychol 1987;12:519-531.
  13. Gwaltney JM, Hendley JO. Transmission of experimental rhinovirus infection by contaminated surfaces. Am J Epidemiol 1982;116:828-833.
  14. Sattar SA, Jacobsen H, Springthorpe S, Cusack T, Rubino J. Chemical disinfection to interrupt the transfer of Rhinovirus type 14 from environmental surfaces to hands. Appl Environ Microbiol 1993;59:1579-1585.
  15. Sizun J, Yu MWN, Talbot PJ. Survival of human coronaviruses 229E and OC43 in suspension and after drying on surfaces: a possible source of hospital-acquired infections. J Hosp Infect 2000;46:55-60.
  16. Smith CB, Purcell RH, Bellanti JA, Chanock RM. Protective effect of antibody to parainfluenza type 1 virus. New Engl J Med 1966;275:1145-1149.
  17. Couch RB. Rhinoviruses. In Fields BN, Knipe DM, Chanock RM, Hirsch MS, Melnick JL, Monath TP and Roizman B, editors. Field's Virology. New York: Raven Press, 1990. p. 607-629.
  18. Reed SE. An investigation on the possible transmission of rhinovirus colds through indirect contact. J Hyg 1975;75:249-258.
  19. Ansari SA, Springthorpe VS, Sattar SA, Rivard S, Rahman M. Potential role of hands in the spread of respiratory viral infections - studies with human parainfluenza virus 3 and rhinovirus 14. J Clin Microbiol 1991;29:2115-2119. 
  20. Hendley JO, Wenzel RP, Gwaltney JM. Transmission of rhinovirus colds by self inoculation. New Engl J Med 1973;288:1361-1364.
  21. Carter CH, Hendley JO, Mika LA, Gwaltney JM. Rhinovirus inactivation by aqueous iodine in vitro and on skin. Proc Soc Experi Biol Med 1980;165:380-383.
  22. Bean B, Moore BM, Peterson LR, Gerding DN, Balfour HH. Survival of influenza viruses on environmental surfaces. J Infect Dis 1982;146:47-51.
  23. Morens DM, Rash VM. Lessons from a nursing home outbreak of influenza A. Infect Cont Hosp Epidemiol 1995;16:275-80.
  24. Hall CB, Douglas R, Geiman JM. Possible transmission by fomites of respiratory syncytial virus. J Infect Dis 1980;141:98-102.
  25. Brady MT, Evans J, Cuartas J. Survival and disinfection of parainfluenza viruses on environmental surfaces. Am J Infect Cont 1990;18:18-23.