A number of viruses. Fairly common ones are enteroviruses, varicella, and mumps virus.
Aseptic. Meningitis is an inflammation of the outer coating of the brain. A spinal tap will show inflammatory cells in spinal fluid. Bacteria will be seen and cultured if present. If nothing grows it is considered aseptic. This usually means that a virus is the cause. Special tests and cultures may or may not identify the virus. Viruses may infect the brain itself causing encephalitis.