How soon until you feel sick after catching someone else's illness? From measles to colds, flu and Covid, our expert guide to when the symptoms will hit...

Sitting next to someone with a hacking cough on a crowded plane or train, hearing the friend you had a drink with last night has since tested positive for Covid, or your child's schoolfriend has thrown up in the classroom, most of us fear the worst and wonder if, or simply just when, we'll come down with the same lurgy. You catch cold or flu by inhaling particles of the virus, usually in a droplet of mucus from someone who is infected, via coughs or sneezes - but also from touching surfaces that have been contaminated, says Professor Eccles But one of the most important factors is the pathogen's reproduction rate, a measure of how contagious it is, based on the average number of secondary infections one person causes. They then burst out and travel around to infect more cells Whether or not an infected person is contagious during the incubation period depends on the type of infection — with measles and chickenpox, for example, the individual may be unaware they are infected and could be spreading the viral droplets to others for weeks before their own symptoms appear. This delayed response is because it takes time for immune cells to bind to the virus and they become stuck in the skin — 'this is when spots appear', says Professor Eccles. 'You catch cold or flu by inhaling particles of the virus, usually in a droplet of mucus from someone who is infected, via coughs or sneezes — but also from touching surfaces that have been contaminated with these droplets and then touching your mouth, nose or eyes,' adds Professor Eccles.