State the term - Viral Infections
A virus is an encapsulated aggregate of nucleic acid that may be made of either DNA or RNA. Some viruses, such as those causing poliomyelitis and rabies, are called neurotropic viruses, because they have a special affinity for cells of the central nervous system. In contrast, pantropic viruses (such as those that cause mumps and herpes simplex) attack other body tissues in addition to the CNS. Most viral infections of the nervous system produce nonspecific lesions affecting widespread regions of the brain, such as lesions due to St. Louis encephalitis, rabies, and poliomyelitis.