A number of different RNA and DNA viruses can invade the brain and cause neurological dysfunction. These range from the tiny polio picornavirus, which has only 7kb of RNA genetic code that preferentially infects motor neurons, to the relatively large cytomegalovirus, which has >100 genes in its 235kb DNA genome and causes various neurological problems in the developing brain but is comparatively harmless to adults. This brief overview of some aspects of neurovirology addresses the complex problems that underlie an appreciation of the contribution of viral infections to brain disease. [This review is part of the INMED/TINS special issue "Nature and nurture in brain development and neurological disorders", based on presentations at the annual INMED/TINS symposium (http://inmednet.com/).]