Pathogens are organisms that are capable of invading living bodies, often causing disease. Pathogens are inherently harmful; however, a new trend has recently emerged suggesting that pathogens could act as potential therapeutic agents. It became increasingly important to candidate pathogens for beneficial use in medicine and biological studies. Cellular barriers and immune system are powerful obstacles; however, pathogens are able to overcome these defenses, and targeting strategies, using genetically engineered pathogens, can reduce potentially damaging effects of the molecule to be delivered. The central nervous system requires more focused studies in this respect, using recently developed techniques in molecular science, such as genome manipulation.