The location within a host organism’s cells where an infection occurs.

Infections can be extracellular (occurring outside the cells in body fluids or on mucosal surfaces, for example) or intracellular (occurring inside cells).

Several intracellular pathogens such as Mycobacterium tuberculosis, Plasmodium falciparum and human immunodeficiency virus manage to evade the host immune reaction and cause disease by replicating inside the host’s cells. Concentrations of antibiotics need to be high enough in the subcellular compartment of an eukaryotic cell where the microorganisms are located.