It depends on the virus that caused the infection. Most viral infections are infectious before you start feeling 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 or two days before your symptoms appear. Symptoms are usually worst 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, symptoms usually last about a week, although a cough may last up to three weeks. Colds tend to last longer in children up to age five, typically around 10 to 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 being exposed to the virus and when symptoms develop). For glandular fever, this can be 33 to 49 days. You’ll remain infectious for at least two months after becoming infected. Some people have the virus in their saliva for up to 18 months after infection. However, glandular fever is not very infectious and the amount of time people remain infectious is variable.
Measles
Symptoms of measles appear 9 to 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 and 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. Find out more about measles symptoms.
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 normally be five to 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 varies, depending on the cause of the infection.
Tonsillitis is most commonly caused by a viral infection, but it can also be caused by bacteria.
Read the answers to more questions about infections.
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