Occasionally central nervous system involvement. Develops in the first week after the appearance of the rash. This is the main reason for vaccination. There is little or no production of virus in the brain but myelin basic proteins are affected and patients T-cells are reactive to myelin, thus it is autoimmune disease. Characterized by spread of measles virus in the brain.
Some neurons contain large accumulation of measles nucleocapsid.
Sequencing reveals many mutations in the M protein, and to lesser extent in other genes. Essentially all primary measles infections give life long immunity. Only one sera-type. Do not find antigen drift. In third world countries it is a major problem.