How long is someone infectious after a viral infection?

The length of time you're infectious for after having a viral infection will depend on the type of virus involved. The infectious period often begins before you start to feel unwell or notice a rash.

The infectious periods for some common viral infections are described below.

Bronchitis

The length of time that bronchitis is infectious varies, depending on its cause. Symptoms may last two to three weeks, although the cough may take longer to go.

Bronchitis is most commonly caused by viruses. It often follows another illness, such as a cold or flu. Bronchitis can also be caused by bacteria, but this is much less common.

Chickenpox

Chickenpox is infectious from about one to two days before the rash appears until all the blisters have fully crusted or scabbed over. This is usually five to six days after the start of the rash.

Common cold

The common cold is infectious about one to two days before your symptoms appear. Symptoms are usually worse during the first two to three days. This is when you're most infectious, although you may still be infectious until your symptoms have gone.

In adults and older children, cold symptoms usually last about a week, although a cough may last up to three weeks. Colds tend to last longer in children up to the age of five, typically around 10-14 days.

Flu (influenza)

Flu is usually infectious a day before symptoms start and remains infectious for about five to six days. Children and people with lowered immune systems may be infectious for a few days longer.

Glandular fever

Glandular fever is infectious during the incubation period (the time between catching the virus and developing the symptoms). For glandular fever, this can be two to four weeks. Some people have the virus in their saliva for up to 18 months after infection. However, glandular fever isn't very infectious and the length of time people remain infectious varies considerably.

Measles

Symptoms of measles appear 9-11 days after you become infected. Measles is most infectious after the first symptoms appear and before the rash develops.

First symptoms of measles include a high temperature, red eyes, sensitivity to light and cold-like symptoms, such as a runny nose, watery eyes, swollen eyelids and sneezing. About three to four days later, a red-brown spotty rash develops that lasts for up to eight days.

Mumps

Mumps causes your salivary glands to swell. These glands are just below and in front of your ears. Mumps is infectious from about six days before your glands swell until about five days after they swell.

Rubella (German measles)

Rubella is infectious for one week before the rash appears and for five days afterwards.

Shingles

Shingles is infectious from when the rash first appears until the last blister has scabbed over. This will usually be between five and seven days after your symptoms start.

Tonsillitis

Symptoms of tonsillitis usually last for approximately one week. However, the length of time that you're infectious will vary depending on the cause of the infection.

Tonsillitis is most commonly the result of a viral infection, but it can also be caused by bacteria.

Read more answers to questions about infections.

Further information:

• How long will I be infectious after starting antibiotics?
• How long do bacteria and viruses live outside the body?
• What are the incubation periods for infections?
• HPA: infections A-Z
• NICE: respiratory tract infections – antibiotic prescribing