This form of parasitism is exhibited by a wide range of organisms that include viruses, bacteria, fungi, and mold. They infect other organisms in order to complete their life cycle. This is so, because the progression of their life stages is highly specific and dependent on the host organisms. Some complete their life cycles within a single host and are said to have direct life cycles (e.g., Necator americanus), whereas few others need at least 2 or more hosts in a particular order, and they are said to possess indirect or complex life cycles (e.g., Plasmodium falciparum). Since the organisms depend on the host cell for nutrition, they must keep it alive and viable for as long as possible till their own life cycle has been completed. This is seen in the case of lytic viral infection of a host cell. The virus infects the cell and takes over the cellular functioning, but it does not harm the cell till it has reproduced and replicated repeatedly. This goes on till the sheer number of the progeny causes the cell membrane to rupture (cell explodes) and release the offspring. A variety of mechanisms are employed by the obligate parasites to invade the host cell and then evade its immune response. Since the parasites are usually smaller in size, they rely on either passive or active uptake by the host cell. Viruses only insert their genetic material in the host cell, where it gets incorporated into the host's genome. Other parasites bear markers on their surface which are similar to the ones on the surface of host cells. Since they evade the immune system, the presence and proliferation of these parasites gives rise to a multitude of health disorders within the host. The infection by an obligate parasite that follows a complex life cycle modifies the cellular chemistry of the host. This in turn allows the parasite to manipulate the behavior of the host in order to facilitate its transfer to the next host required to complete its life cycle. This is observed in the case of Toxoplasma gondii. It requires two hosts for the completion of its life cycle. Any warm-bodied animal can be used as the initial host during its asexual phase. Mostly rats are carriers of this parasite since they live in sewers and ingest fecal matter. The parasite induces the production of a chemical within the rat that causes the rat to be attracted to (instead of being repelled by) the odor of cat urine. This attraction causes it to seek out places populated with cats, and cat being the natural predator of rat, preys on the infected rat while also consuming the parasite. Here, in the cat's system, the asexual parasite completes the sexual part of its life cycle.

✱ All viruses are obligate intracellular parasites as they cannot reproduce outside the host cell, e.g., bacteriophages, AIDS virus (HIV), human papilloma virus (HPV), tobacco mosaic virus (TMV), etc., at all. ✱ Sarcobium lyticum infects small amoebae. ✱ Chlamydia trachomatis causes a sexually-transmitted disease of the same name as its genus, in humans. It also causes nongonococcal urethritis, pelvic inflammatory disease, and trachoma. ✱ Rickettsia prowazekii infection causes typhus fever in humans. ✱ Inhalation of Mycobacterium tuberculosis causes tuberculosis, a disease that affects the lungs. ✱ 14 genera of microsporidia parasites infect a variety of fish. For example, Glugea infects flounder, Loma infects Atlantic cod, salmon, and rainbow trout, Nosemoides infects cichlids, etc. ✱ Pneumocystis jirovecii infects humans and causes pneumonia. ✱ Plasmodium falciparum causes malaria in humans. ✱ Leishmania spp. cause leishmaniasis in humans.