When are you contagious with a cold or the flu? What shedding studies show

The short answer for influenza is that you are contagious for roughly a week, and you can be spreading the virus before you feel unwell — sometimes without ever feeling very unwell at all. The precise timeline has been measured, and it explains why "just stay home once you feel sick" catches the illness later than transmission actually begins.

The influenza timeline, measured

The most detailed picture comes from volunteer challenge studies, in which healthy people are deliberately given a controlled dose of wild-type influenza virus and then sampled every day. A review pooled 56 such studies with 1,280 participants and reconstructed the average course of infection [s1]. Virus shedding — the release of infectious virus, and the thing that makes a person contagious — increased sharply between 0.5 and 1 day after infection and consistently peaked on day two [s1]. Averaged across 375 participants, shedding lasted 4.80 days (95% confidence interval 4.31 to 5.29) [s1].

The revealing part is how that lines up with symptoms. Total symptom scores did not rise until day one and peaked on day three, a day after viral shedding had already peaked [s1]. In other words, a person is often at their most infectious around the time symptoms are just starting, before the illness reaches its worst. Waiting until you feel clearly sick to withdraw from other people misses the front end of the contagious window.

You can spread it with few symptoms, or none

Not everyone who catches influenza feels it. In the same challenge review, 66.9% (95% CI 58.3 to 74.5) of infections were symptomatic — which means around a third produced no recognised illness despite an active infection [s1]. A community study in Hong Kong followed household members of people with confirmed influenza and identified 235 secondary infections; of these, 31 (13%) were paucisymptomatic, with a single symptom, and 25 (11%) were fully asymptomatic [s2]. So close to a quarter of infected household contacts had one symptom or none.

Those milder infections were less contagious, but not harmless. Viral shedding in the asymptomatic and paucisymptomatic cases was shorter, declined more rapidly, and ran roughly 1 to 2 log10 copies lower than in symptomatic cases [s2]. Lower is not zero: people who barely feel ill still carry and can pass on the virus, which is why a workplace or household outbreak can spread through people who never thought they were sick.

The common cold is fuzzier

Colds are harder to pin to a single clock because they are not one disease. Among the many viruses that cause the common cold, rhinoviruses are the most prominent, and about a quarter of all colds are still without a proven cause [s3]. That heterogeneity means there is no single, tidy shedding curve for "a cold" the way there is for influenza; different viruses follow different courses. The general pattern people report — symptoms building over the first couple of days and easing over a week or so, with the early days the most infectious — is a reasonable rule of thumb, but it is a looser one than the influenza data support, and the underlying virus is rarely identified in everyday illness. It is also why "it is just a cold, so it does not matter" is a weak guide to when to be careful: the same early-days infectiousness applies, and the person you pass a rhinovirus to may not tolerate it as easily as you did.

What this means in practice

The evidence describes averages from challenge experiments and one community cohort; it does not tell any individual exactly when they stopped being infectious, and it is not a clearance test. Two practical implications follow from the data rather than from advice. First, because influenza shedding leads symptoms, the window in which precautions matter opens before you feel your worst — which is the case for masking, distance and hand hygiene around others in the first days of any respiratory illness. Second, feeling only mildly unwell does not mean you are not contagious, since a large share of infections are mild or silent yet still shed virus. Anyone who is unwell, caring for someone vulnerable, or unsure whether they are past the infectious period should follow local guidance and, where relevant, a clinician's advice rather than a fixed number of days.