Meningitis can be caused by both viral and bacterial infections. Viruses are the most common cause of meningitis, with enteroviruses being the most frequent viral cause in the United States. Bacteria such as Streptococcus pneumoniae, Neisseria meningitidis, and Listeria monocytogenes are some of the leading bacterial causes of meningitis. Rarely, fungi or parasites can also cause meningitis.