The most common causes of viral [[meningitis]] are:
Enteroviruses

Coxsackie or Echovirus groups of enteroviruses are the most common cause of viral meningitis. Most infections produce no symptoms, or mild symptoms such as sore throats, colds and flu-like illnesses. Some of them may also cause stomach upsets and diarrhoea. Enteroviruses mostly affect children, who are the main transmitters of these viruses. Enteroviral infections are prevalent in the UK during summer and autumn months. Spread is mainly by hand to mouth transmission.

Herpes viruses

Herpes viruses are the other common cause of viral meningitis in adolescents and adults in developed countries.

Herpes viruses include the herpes simplex viruses (HSVs) and varicella zoster virus – the same virus that causes chickenpox and shingles.

HSVs can cause meningitis or encephalitis (inflammation of the brain itself, which is much more serious). HSV encephalitis is mainly caused by HSV-1 (which is also the cause of most cold-sores), whereas meningitis is more often caused by HSV-2 (the cause of most genital herpes). HSV viruses have been linked to Recurrent Lymphocytic Meningitis, also known as [[Mollaret’s meningitis]]. It is characterised by sudden attacks of meningitis symptoms that usually last for 2-7 days and are separated by symptom free intervals lasting for weeks, months or years.

HSV-1 and -2 infections are very common but most people affected have only minor symptoms or no symptoms at all. Transmission is via contact with an infected area of the skin when the virus is active. HSV-1 is usually acquired orally during childhood and about 6 out of 10 people in the UK carry it. HSV-2 is primarily a sexually transmitted infection and is carried by about 1 in 10 of the UK population[3]. Many people harbour HSV without ever knowing they have it. Since they can transmit the virus without having symptoms, infection can be spread unknowingly to contacts. The timing of transmission is unpredictable, so genital herpes infection (and herpes meningitis) can appear even after many years in a monogamous relationship.

VZV meningitis can occur at the same time as chickenpox or shingles or it can occur in its own without any rash or skin manifestation.

Other Herpes viruses are less frequent causes of meningitis compared to HSVs.

Mumps and measles
Before MMR vaccine was available, mumps was the most common cause of viral meningitis in the UK and occurred in 15% of patients with mumps[4]. A recent resurgence in cases in the UK largely affected late teenagers and young adults who did not receive a full course of measles-mumps-rubella (MMR) vaccine[1]. Measles can also cause meningitis and encephalitis.
Flaviviruses
These viruses normally affect wild animals such as birds or rodents and are carried by mosquitoes or ticks. Some of these viruses such as West Nile virus, Japanese B encephalitis and tick-borne encephalitis can cause meningitis or encephalitis in humans but this is not a problem in the UK and Ireland.

Human immunodeficiency virus (HIV) can cause meningitis during the early stages of HIV infection. In cases of viral meningitis it is not always possible to identify the type of virus responsible for the disease (approximately 30-40% of patients have no pathogen identified). Although HIV is a rare cause of viral meningitis, it is important that adults with viral meningitis due to unknown cause undergo an HIV test. This is because viral meningitis may be the first sign of infection with HIV and if it is not identified at this early stage then it may remain symptomless and undiagnosed until the individual has advanced disease or AIDS. It is hoped that by screening such cases more individuals with HIV will be identified and treated early resulting in better outcomes.