Computer viruses infect a variety of different subsystems on their hosts. A computer virus is a malware that, when executed, replicates by reproducing it self or infecting other programs by modifying them. Infecting computer programs can include as well, data files, or the boot sector of the hard drive. When this replication succeeds, the affected areas are then said to be “infected”.

References: https://en.wikipedia.org/wiki/Computer_virus