Final answer:
Protozoan cysts and oocysts are protective stages in the life cycles of some protozoans, allowing them to survive harsh conditions and spread to new hosts.
Step-by-step explanation:
Protozoan cysts and oocysts are stages in the life cycle of certain protozoans, which are single-celled eukaryotic organisms. These structures provide a protective barrier that allows the protozoan to survive in harsh external environments. Specifically, a cyst is a dormant form with a protective wall that enables the protozoan to withstand adverse conditions; this process is known as encystment. When conditions improve, the cyst undergoes excystment, transitioning back to the active trophozoite stage. Furthermore, oocysts are a form of cysts important in the life cycle of certain parasites like Toxoplasma gondii, where they transform into active forms called tachyzoites that localize in the host's tissues. Protozoan organisms, such as Giardia or Plasmodium species, are common human parasites, sometimes spreading through cysts in contaminated water or food.