The length of time you're infectious for after having a viral infection depends 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.
The length of time that bronchitis is infectious varies, depending on its cause. In most cases, bronchitis is caused by the same viruses that cause the common cold or flu and you're likely to be infectious as long as you have cold or flu symptoms.
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.
The common cold is infectious from a few days before your symptoms appear until all of the symptoms are gone. Most people will be infectious for around two weeks.
Symptoms are usually worse during the first two to three days and this is when you're most likely to spread the virus.
Flu is usually most infectious from the day your symptoms start and for a further three to seven days. Children and people with lowered immune systems may be infectious for a few days longer.
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 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 isn't very infectious and the length of time people remain infectious varies considerably.
Symptoms of measles appear around 10 days after you become infected. Measles is most infectious after the first symptoms appear and before the rash develops.
First symptoms of measles include:
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 afterwards.
Rubella is infectious for one week before the rash appears and for up to four days afterwards.
You should stay away from school or work for five days after the rash starts to avoid infecting others and try to avoid contact with pregnant women during this time.
Shingles is infectious from when the rash first appears until the last blister has scabbed over. This is usually after about 10-14 days.
Tonsillitis itself isn't contagious but the viruses that cause it are. The length of time you're infectious will depend on the virus. Read more on the causes of tonsillitis.
When a person is 'infectious', it means they're able to pass their infection on to others.
You're usually no longer infectious 24 hours after starting a course of antibiotics, although this time period can sometimes vary.
For example, the antibiotics may take longer to work if your body takes longer to absorb them or if you're taking other medication that interacts with the antibiotics.
Talk to a pharmacist or doctor if you have any questions or concerns about your course of antibiotics.
It's important to finish your course of antibiotics, even if you're no longer infectious and feeling better.
Failing to finish the course may result in the infection returning.
Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.