Causes of a cold
There are many viruses that can cause colds, sometimes seasonally and sometimes in epidemics. It is estimated that up to 50% of colds are caused by one of the more than 100 rhinoviruses (rhino =nose).
Other viruses that cause colds are the entroviruses, coronaviruses, adenoviruses, respiratory syncytial virus and parainfluenza viruses. Some of these viruses are capable of causing more severe disease in very young infants (such as pneumonia), but only cause colds in older children and adults.
A person with a cold is usually contagious from 24 hours before symptoms start and as long as they last, which is usually about a week.
Rhinoviruses are most often spread by direct contact with infected secretions, e.g. touching objects such as handkerchiefs, door-knobs or eating utensils that a person with a cold has touched before, and then touching one's nose or mouth.
Rhinoviruses are less often spread by airborne particles, such as when an infected person sneezes.
Your immune system responds by attacking the virus with white blood cells. If your immune system cannot recognise the virus from a previous infection, the response is "non-specific", meaning your body produces as many white blood cells as possible and circulates them to the infected sites.
White cells produce chemicals to kill virus-infected cells, and this is what causes the nasal inflammation and swelling, increased mucous secretions and the general feeling of achiness.
Once infected with a specific cold virus, the body develops immunity to it in the form of "memory white cells" and antibodies, which will control the virus quickly in the event that it is encountered again. Immunity will prevent another cold being caused by the same rhinovirus for some months at least, but does not protect against others.
Reviewed by Dr Marvin Hsiao MBBCH MMed MPH, Division of Medical Virology, Department of Clinical Laboratory Sciences, University of Cape Town. February 2015.
(Previously reviewed by Dr Eftyhia Vardas, University of the Witwatersrand 2011)