Aseptic meningitis commonly refers to viral meningitis but more accurately refers to patients with meningeal pleocytosis with negative bacterial cultures. This can arise due to viral infection but also fungal infection or mycobacterial infection, or as a result of a drug reaction or a parameningeal foci or malignancy.

Numerous viruses can be responsible, but a few of them are of particular interest because of the prognostic and therapeutic implications. However, viral etiology frequently remains unidentified.

Acute viral meningitis is characterized by elevated white blood cell count (WBC) in the cerebrospinal fluid (CSF) with lymphocytic predominance; it is usually < 1000 cells/µL. Proteins in the CSF are usually only slightly elevated (usually < 250 mg/dL), while glucose is normal. Presence of red blood cells (RBCs) is unusual and may indicate another diagnosis in the correct context, such as subarachnoid hemorrhage or herpes simplex virus encephalitis.

The condition typically manifests with fever and signs of meningeal irritation, including headache, sensitivity to light, and stiff neck. Mental status is usually altered, ranging from somnolence to coma. Individuals at the extremes of age and immunocompromised patients may not always present with the usual features: newborns may only have a fever without other symptoms; the elderly may only have mental status changes. 

Rarely, meningitis may persist for 4 weeks and is then considered chronic meningitis.
    
The most frequently identified cause of acute viral meningitis is enteroviruses, which are members of the Picornaviridae family and include echoviruses and coxsackie viruses. These enteroviruses are more common in summer and fall but may be seen year-round in warmer areas. Transmission is fecal-oral. All age groups may be affected, but infants and young children are usually more susceptible, and neonates in particular can develop a sepsis-like multiorgan failure. Extraneurologic symptoms such as vomiting, diarrhea, rhinorrhea, sore throat, or cough may precede or accompany the neurologic manifestations. An exanthematous rash is a nonspecific accompanying manifestation. Certain enteroviruses may be associated with the more specific hand-foot-and-mouth disease (HFMD).

The family of herpesviruses is much less frequently implicated in acute viral meningitis, but it is important to maintain a high index of suspicion because of the possibility of associated potentially fatal encephalitis and the availability of treatment for this complication. Herpes simplex virus type 2 (HSV-2) is implicated in meningitis more frequently than HSV-1 and is usually, but not always, associated with recent or concomitant genital ulcers, especially in women. HSV-2 is also known to cause recurrent aseptic meningitis, known as Mollaret's meningitis, which may not necessarily be associated with genital ulcers. HSV-1 is more commonly implicated in encephalitis, especially that with temporal lobe involvement. In the family of herpesviruses, varicella zoster virus (VZV) (not necessarily with skin lesions), human herpes virus (HHV) type 8 (in infants), Epstein-Barr virus (EBV), and cytomegalovirus (CMV) (especially in immunocompromised individuals) are also possible etiologies.

A broad variety of other viruses have also been implicated in acute viral meningitis. Of particular interest, HIV may cause meningitis or a meningoencephalitis during the primary infection or, less frequently, later. Mumps is a concern in the unimmunized individual. Mosquito- and tick-borne viruses, such as West Nile virus or St. Louis encephalitis virus, may also cause isolated meningitis. Lymphocytic choriomeningitis virus (LCMV) is a consideration when contact with rodents or hamsters is reported.