Viruses

noun

1. A submicroscopic, non-cellular structure consisting of a core of DNA or RNA surrounded by a protein coat, that requires a living host cell to replicate, and often causes disease in the host organism.
2. A disease caused by these organisms.

   "He caught a virus and had to stay home from school."

3. Venom, as produced by a poisonous animal etc.
4. A type of malware which can covertly transmit itself between computers via networks (especially the Internet) or removable storage such as disks, often causing damage to systems and data; also computer virus.
5. Any type of malware.