Asked by: Neal Gilbody, Swaffham
The common cold is an upper respiratory tract infection that can be caused by at least five different types of virus. The most common of these is the rhinovirus, accounting for more than half of infections. Each virus type has many different strains, or ‘serotypes’, and there are well over a hundred serotypes of rhinovirus alone.
From around four days after infection, the immune system produces antibodies to tag and destroy the virus particles and infected cells, but these are highly specific to that particular virus serotype.
In theory, two different strains could infect you simultaneously, each requiring a separate immune response. However, the second ‘cold’ could also be a separate, opportunistic co-infection.
Damage to the lining of the nose or throat and an already-stressed immune system can allow viruses and bacteria that would otherwise have been resisted to gain a toehold. Bronchitis, pneumonia, otitis media and strep throat can all begin in this way.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.