A computer is considered to be “infected” when it has been hacked in such a way so as to follow instructions of the hacker rather than the computer’s owner. An infected computer may be part of a botnet [which can be used for Distributed Denial of Service (DDoS) attacks], or it just periodically show ads or redirect a user’s web browser. While it refers to a more specific action than “hacked”, it also furthers the analogy of computer security as a medical analogy (i.e., the idea that one “catches” computer viruses). (There has been some concern in the academic literature about the ways in which the medical analogy can mislead lay people into bad computer security practices).
Source: Election Cybersecurity 101 Field Guide – Glossary, Center for Democracy & Technology, https://cdt.org/insight/election-cybersecurity-101-field-guide-glossary/