How long is flu contagious?

The NHS puts it more simply: you are more likely to pass flu on in the first five days. Most people recover within about a week.

That answer hides two questions. 'How long is flu contagious?' is about the risk to other people, counted from the day your symptoms began. 'How long does flu last?' is about you: a few days to under two weeks, with a cough that outlasts everything else. The NHS and the CDC are the main sources below, and where their figures run out it says so.

Day by day

Counting the day symptoms start as day 0, this is how the UK & USA (NHS / CDC) figures fall.

Are you contagious before flu symptoms start?

Yes, for about a day. The CDC says influenza viruses can be detected in most infected people from one day before symptoms develop. The incubation period, the gap between catching the virus and the first symptom, is typically about two days, with a range of one to four. On a normal timeline you catch flu on Monday, feel fine on Tuesday while already able to pass it on, and go down with it on Wednesday.

The NHS puts no number on the days before symptoms. Its flu page says only that you are more likely to give flu to others in the first five days after getting infected, which counts from infection rather than from symptoms. The ECDC, the European agency, agrees with the CDC: the virus is found in nose and throat samples from one day before symptoms begin.

When is flu most contagious, and why?

The CDC says people with flu are most contagious during the first three days of their illness: day 0, the day symptoms start, then day 1 and day 2. The NHS draws a slightly wider line at the first five days after getting infected. Both agree on the shape: the risk to others is front-loaded, highest while you feel worst and falling as the fever goes.

The reason is how flu travels: mainly in droplets made when you cough, sneeze or talk, which land in the mouths and noses of people nearby or are breathed in, the CDC says. Less often you pick it up from a surface and then touch your mouth, nose or eyes; the NHS says germs from coughs and sneezes can live on hands and surfaces for 24 hours. If you can only keep away from other people for a few days, days 0 to 2 are the ones.

How long does flu last, and when does it stop being contagious?

Two clocks run at once. The contagious clock: the CDC says the virus can be detected up to five to seven days after you fall ill, so most healthy adults are no longer a risk to others by about day 5, and almost all by day 7. The illness clock: the WHO says most people recover from fever and other symptoms within a week, and the CDC says a few days to less than two weeks.

The cough is the exception. The WHO says the flu cough can be severe and last two weeks or more; the ECDC says coughing, tiredness and feeling unwell can last some weeks. A cough in week two or three falls well outside the five-to-seven-day window in which the CDC and the ECDC say the virus is usually still found. Still feeling wrung out is not the same as still passing flu on.

Neither the NHS nor the CDC names a single safe day. The NHS works from symptoms: stay at home while you have a high temperature, feel hot, cold or shivery, or do not feel well enough for your normal activities. The CDC adds a threshold: 24 hours of improving symptoms with no fever and no fever-reducing medicine, then five more days of precautions, because you may still be able to spread the virus after you feel better.

What a flu test tells you, and what it does not

A flu test answers one question: is it flu? The CDC describes three kinds. Rapid antigen tests give a result in about 10 to 15 minutes but may be less accurate; rapid molecular tests take 15 to 20 minutes and are more accurate; laboratory PCR takes one to several hours. A negative rapid test does not rule flu out, and in flu season a doctor may diagnose you from your symptoms without testing.

No test tells you when you have stopped being contagious. The CDC's testing page says nothing about contagiousness, and neither the NHS nor the CDC bases its return-to-work advice on a test result; both work from symptoms and days.

Do antivirals make you less contagious?

Antiviral tablets for flu (oseltamivir is the best known) work best when started within two days of the first symptoms. The CDC says they can shorten the illness by about a day and reduce some complications; the ECDC gives the same 48-hour window. They are recommended for people at higher risk of serious flu, such as those with asthma, diabetes or heart disease, not for everyone.

On contagiousness, the official pages are silent. The CDC's treatment page does not say that antivirals shorten the time you can pass flu on, and the NHS flu page does not mention them at all. If you are taking them, count your contagious days from your first symptom, not your first tablet. Antibiotics only work if you have a bacterial infection, the NHS says, and flu is caused by a virus.

When can you go back to work, school or nursery after flu?

The NHS rule works from symptoms and has no day count: try to stay at home and avoid contact with other people if you have a high temperature, feel hot, cold or shivery, or do not feel well enough to do your normal activities. For schools and nurseries the UK Health Security Agency's exclusion list agrees: a child with flu stays away until they are better and no longer have a temperature.

The CDC offers a simple check for the morning you want to go back: for at least 24 hours your symptoms have been getting better overall and you have had no fever without fever-reducing medicine. It then asks for five more days of precautions, such as a well-fitting mask, keeping your distance and airing rooms, because you may still be able to spread the virus.

Neighbouring countries draw the line differently. Germany's BZgA asks for three to five days at home and until symptoms have improved. Italy's ISS says stay off until recovered, about seven days. The Dutch RIVM applies no formal exclusion and notes that the risk has largely dropped by day 5. France's Ameli and the Spanish regional guidance publish no return rule at all.

Everyday situations: newborns, grandparents, beds, gyms and flights

In each case the day count starts on the first day of symptoms.

• A newborn or a pregnant partner at home: the WHO names children under five, pregnant women and older people among those at greater risk of severe disease, and the NHS lists pregnancy and age 65 and over as reasons to seek urgent advice about flu. Keep the sick person out of the baby's room for the CDC's three-day peak at least, and ideally until day 7, and wash your hands before every contact.
• Sharing a bed: flu spreads in droplets from coughs, sneezes and speech to anyone close by, so a shared bed in the first three days is about the worst case. If there is a spare room, use it for days 0 to 2 at least.
• Grandparents visiting: over-65s are on the NHS list of people who need urgent advice if they catch flu. Wait until the sick person has been fever-free and improving for 24 hours, and past day 5 if the grandparent is frail.
• The gym or a flight: the NHS says stay at home while you do not feel well enough for your normal activities, and neither the NHS nor the CDC publishes a rule for flying. Apply the CDC's 24-hour threshold first, and wear a well-fitting mask if the trip falls inside its five-day precaution window.
• A family dinner: day 0 to day 2 is the CDC's peak, so a dinner in that window with an older relative is the one to cancel. After day 5 most healthy adults are past the point at which the virus is usually still found.
• A care-home visit: neither the NHS flu page nor the UKHSA exclusion list sets a rule for visitors, so homes set their own. The CDC's five days of precautions after you return to normal activities is a sensible minimum.

How long is flu contagious in children, older adults and people with weakened immunity?

The CDC says some people, including young children and people with weakened immune systems, may be contagious for longer, but it gives no number. The ECDC says young children, older people and those with weakened immune systems go on shedding the virus for somewhat longer, again without a figure. The NHS flu page says nothing on the point, so for the UK there is no official day on which a child stops being contagious.

Other European authorities do put numbers on it, and the spread is wide. France's Ameli says children can remain contagious up to seven days. The Dutch Thuisarts says up to 14 days, and the RIVM says people with a weakened immune system can shed the virus for weeks and in some cases months. Treat those as the documented outer limit, not as a UK rule. No authority says older adults stay contagious for longer; they are simply the people most likely to be harmed, which is why the NHS asks anyone aged 65 or over with flu symptoms to seek urgent advice.

Why the official guidance on flu differs between countries

On the biology, Europe and the US broadly agree. The ECDC, the CDC and the Dutch RIVM all put the virus in the nose and throat from about a day before symptoms to five to seven days after, and Germany's RKI gives five days as typical and seven at the outside. Italy's ISS is the outlier at both ends: contagious from two days before symptoms, and up to ten days after.

Incubation is where they part company. Spain's Madrid regional service and Italy's ISS say one to two days; the CDC, Ameli, the Institut Pasteur and the RIVM say one to four, typically two; the RKI says one to three. These are different readings of the same research, not different viruses.

Return rules differ most because they answer different questions. The NHS and the UKHSA write for the individual and use symptoms. Germany's BZgA writes for the public but wants a number, and says three to five days. The RIVM writes for public-health professionals and concludes that formal exclusion is not worth it once the risk has dropped by day 5. Ameli and the Spanish guidance say nothing. Each draws the line for a different reader.

What the guidance says in six countries

The same illness, six official positions. Where a country publishes nothing, the table says so rather than borrowing a neighbour's number.

How to stop flu spreading at home

• Wash your hands often with warm water and soap; the NHS puts this first.
• Cover your mouth and nose with a tissue when you cough or sneeze, and bin used tissues as quickly as possible, the NHS says.
• Wipe surfaces the sick person touches: the NHS says germs from coughs and sneezes can live on hands and surfaces for 24 hours.
• Keep your distance: the CDC says physical distance from others can help lower the risk of spreading a respiratory virus.
• Air the room. The UKHSA's respiratory-infection guidance says to ventilate rooms you have been in by opening windows and leaving them open for at least 10 minutes after you have left.
• Wear a well-fitting mask near others and keep away from anyone at higher risk; both are CDC precautions for the five days after you return to normal activities.

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Other illnesses

Contagious periods for the other illnesses covered by this tool.

This tool gives general information based on published guidance. It does not diagnose you and does not replace advice from a doctor who knows your situation. You choose the illness; we do not work out what you have.