There have now been 15 confirmed cases of measles at a Chichester school in recent weeks, prompting Public Health England (PHE) to set up special clinics.
But could you spot the signs of measles?
Watch our video for some of the main symptoms.
Yasmin Maskatiya, head teacher at Chichester High School School, said: “We have been concerned about the cases of measles and we understand this can be a worrying time for our families, children and staff.
“We wish the children who have contracted measles a speedy recovery to good health.”
The initial symptoms of measles develop around 10 days after a person is infected, according to PHE. These can include:
cold-like symptoms, such as a runny nose, sneezing, and a cough
sore, red eyes that may be sensitive to light
a high temperature (fever), which may reach around 40C (104F)
A few days later, a red-brown blotchy rash will appear. This usually starts on the head or upper neck, before spreading outwards to the rest of the body.
For more information please visit the NHS website.