Encephalitis (encephalitis) is an inflammatory lesion of the brain parenchyma caused by the attack of pathogens. There is a broad and narrow definition depending on the scope of the etiology. The narrow sense refers to inflammatory changes caused by direct invasion of the brain parenchyma by pathogenic microorganisms. The narrow concept is usually used. The vast majority of etiologies are viral, but can also be caused by infections with bacteria, mycobacteria, spirochetes, rickettsiae, parasites, and some may be metaplastic, such as acute disseminated encephalomyelitis. Usually the so-called encephalitis refers mostly to viral encephalitis and post-infectious encephalomyelitis belonging to acute disseminated encephalomyelitis. It is classified as acute, subacute, or chronic according to the course of the disease; bacterial, fungal, or viral according to the pathogenic microorganism; and epidemic or sporadic according to the prevalence. Encephalitis can develop in different genders and ages, and is mostly acute or subacute. It is characterized clinically by high fever, headache, vomiting, coma, and convulsions, and is mostly accompanied by changes in cerebrospinal fluid composition. Prevention and treatment should be based on different etiologies. Meningitis (meningitis) refers to diffuse inflammatory changes in the soft meninges. It is caused by invasion of the soft meninges and spinal cord membranes by various biologic pathogens such as bacteria, viruses, fungi, spirochetes, protozoa, rickettsiae, tumors and leukemia. Bacterial meningitis is a particularly serious disease that requires prompt treatment and, if not treated promptly, may result in death or permanent brain damage within hours. Viral meningitis is more severe but most people recover completely, with a few remaining sequelae. Meningitis can involve the dura mater, arachnoid and soft meninges. Dural meningitis is most often secondary to a cranial infection. Since the widespread use of antibiotics, the incidence of dural meningitis has decreased considerably. Soft meningitis, however, is quite common and includes inflammation of the arachnoid and soft meninges. Therefore, meningitis is now actually referred to as soft meningitis. The vast majority of meningitis is caused by pathogens, with epidemic meningitis caused by Dictyococcus meningitidis being the predominant type; a few are caused by irritating chemicals (e.g., procaine, methotrexate). There are 3 basic types of meningitis: septic meningitis, lymphocytic meningitis (mostly caused by viruses), and chronic meningitis (which can be caused by Mycobacterium tuberculosis, Syphilis spirochetes, Borrelia burgdorferi, and fungi).