Especially replication within the cytoplasm or nucleus of another organism and in a manner that overall results in harm to that other organism.
Intracellular parasites consist predominantly of bacterial pathogens as well as viruses. They also include what can be described as obligatory intracellular parasites.
See also bacterial obligate intracellular parasites (etiologies) as well as tissue invasion without toxin production.
For a microorganism to be a successful it must be able to enter a cell without killing that cell, since alternative it would be acting as a saprophyte rather than parasite. This entrance an occur either via active mechanisms effected by the parasite or instead by the .
In the latter case, this typically occurs via endocytosis, such as phagocytosis, with that process then by the parasite in such a way that the parasite ends up harming the cell rather than, as otherwise can often be the case, the host cell harming the parasite.