All protozoans are found in a life stage called trophozoites. The “troph” stage is actively feeding, mostly motile and responsible for symptoms in a host. Some protozoans can form a cyst stage – a resting, inactive stage that protects the organism because of a thick wall that is produced. These cysts enable the protozoan to survive harsh environmental conditions outside of a host and ensure the organism will be passed to other hosts. Ingestion of the cyst in contaminated food or water is a common method of acquiring many protozoan infections.

Many protozoan parasites require vectors for transmission to occur. A vector is a transport mechanism. With parasites requiring a vector, the vector is a living thing, often an arthropod species (e.g. mosquito).

The life cycles of parasites are very specific. Parasites require transmission to specific species of organisms (the hosts) in a specific order to continue spreading and reproducing. At times, a parasite may be transmitted to a host that is not in their life cycle. This accidental host can experience symptoms of parasite infection, but the parasite is unable to continue its life cycle and continue reproduction.

In additional to an accidental host, host species can also be classified as either the definitive host or the intermediate host:

Protozoan parasites can undergo sexual reproduction, asexual reproduction, or both sexual and asexual reproduction depending on the species. Asexual reproduction will typically involve cell divisions to produce new individuals, whereas sexual reproduction will involve production and fusion of gametes ("male" and "female" cells that have half the DNA of the other cells and can fuse to produce genetically unique individuals).