An oubreak of mumps is circulating among young people across the Horsham area.
Now health specialists are warning people to check that they are up to date with their MMR vaccinations.
Public Health England says there have been 18 confirmed cases across Surrey and Sussex since Christmas.
And the health body is now advising anyone who suspects that they or a family member has mumps should seek medical advice and stay away from lessons, work, exams and social gatherings for five days.
Mumps is most recognisable by painful swellings at the side of the face under the ears.
A spokesman for Public Health England said:”Anyone who is unsure that they have had two MMR vaccinations should contact their GP surgery to check their immunisation status.
“Parents are also encouraged to check that their child has had two doses of the vaccination against measles, mumps and rubella, and if not to make arrangements for the vaccination with their GP surgery.”
Public Health England consultant Dr Margot Nicholls said: “Mumps is circulating and can spread quickly from person to person which is why we often see cases in close knit communities such as schools, universities and colleges.
“Symptoms usually begin with a headache and fever, followed by a swelling of glands in the neck.
“Although most people usually recover without treatment, in some cases it can cause complications such as inflammation of the testicles or ovaries, and in rare cases, meningitis and deafness.
“We are encouraging anyone who is not sure if they have had two doses of the MMR vaccine to arrange to complete their vaccination with their GP as soon as possible.
“Vaccination is the safest and most effective way to protect against the infection, especially when we know it is circulating in the community.
“It prevents most, but not all, cases of mumps and even if a vaccinated person does get mumps, they will likely have less severe illness than an unvaccinated person.
“The MMR vaccination also protects against two more serious illnesses German measles (rubella) and measles.”
For more information visit: www.nhs.uk/Conditions/Mumps/Pages/Introduction.aspx