Viral meningitis is common, but many cases go unreported, say researchers in this week's BMJ.
And due to the increasing incidence of genital herpes in the UK, cases of meningitis caused by the herpes simplex virus are set to rise, they warn.
In 2005-6, 2898 people were admitted to hospital in England with a diagnosis of viral meningitis, yet this is 10 times the number of cases notified to the Health Protection Agency in both England and Wales over the same period.
Meningitis is an inflammation of the membranes that cover the brain and spinal cord and is most common in young children. Viral meningitis generally has a good prognosis, but bacterial meningitis is life-threatening and requires prompt treatment.
However, the symptoms are similar (fever, severe headache, nausea and vomiting, dislike of light and a stiff neck) and they cannot reliably be differentiated, so all suspected cases should be referred to hospital.
Enteroviruses are the most common cause of viral meningitis at all ages, and infants and young children are most susceptible. No specific antiviral treatment is available.
Herpes simplex virus now ranks second among the causes of viral meningitis in adolescents and adults and is set to increase as the incidence of genital herpes rises.
Other causes include varicella zoster virus (the cause of chickenpox and shingles), HIV and mumps. Before widespread immunisation, mumps was a common cause of meningitis. The recent epidemic among young adults was associated with more than 100 cases of mumps meningitis in England in 2004-6.