The duration you are infectious for after having a viral infection relies 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 period that bronchitis is infectious varies, depending on its cause. In most cases, bronchitis is caused by the same viruses that cause the common flu or cold and you’re likely to be infectious as long as you have flu or cold symptoms.

Chickenpox

Chickenpox is infectious from about 1 to 2 days before the rash appears until all the blisters have fully crusted or scabbed over. This is usually 5 to 6 days after the start of the rash.

Common cold

The common cold is infectious from a few days beforehand your symptoms appear until all of the symptoms are disappeared. Most people will be infectious for around 2 weeks.

Symptoms are usually worse during the first 2 to 3 days and this is when you’re most likely to spread the virus.

Flu (influenza)

Flu is frequently most infectious from the day your symptoms start and for a further 3 to 7 days. People and children with lowered immune systems may be infectious for a few days longer.

Glandular fever

Glandular fever is infectious during the incubation period (the phase between catching the virus and developing the symptoms). For glandular fever, this can be 2 to 4 weeks.

Certain people have the virus in their saliva for a few months after recovering from glandular fever, and may continue to have the virus in their saliva on and off for years. However, glandular fever is not very infectious and the period people remain infectious differs considerably.

Measles

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

First symptoms include:

  • high temperature
  • red eyes
  • sensitivity to light
  • cold-like symptoms – like runny nose, sneezing, watery eyes and swollen eyelids

Around 2 to 4 days later, a red-brown spotty rash develops that normally fades after about a week.

Mumps

Mumps causes your salivary glands to swell. These glands are just below and in front of your ears. Mumps is most infectious from a few days before your glands swell until a few days after.

Rubella (German measles)

Rubella is infectious for 1 week before the rash appears and for up to 4 days afterwards.

You should stay away from work or school for 6 days after the rash starts to avoid infecting others and try to avoid contact with pregnant women during this time.

Shingles

Shingles is infectious from when the rash first appears till the last blister has scabbed over. This is usually after about ten to fourteen days.

Tonsillitis

Tonsillitis itself is not contagious but the viruses that cause it are. The period you’re infectious will depend on the virus.