The term "infected cells" refers to cells that have been invaded and taken over by a virus or other pathogen. During an infection, the infecting agent (such as a bacterium, fungus, parasite, or virus) enters the cell and uses its machinery to replicate itself and spread throughout the body. This process can damage or kill the host cells, leading to various health problems. In some cases, infected cells may be visible under a microscope as they reproduce the pathogen.