Here's why:

* Apicomplexans are characterized by a specialized structure called an apical complex which is used for penetrating host cells. This makes them very well adapted to a parasitic lifestyle.

* Many well-known parasites belong to this phylum, including:

* Plasmodium (causes malaria)

* Toxoplasma gondii (causes toxoplasmosis)

* Cryptosporidium (causes cryptosporidiosis)

* Babesia (causes babesiosis)

While other protozoan phyla contain some parasitic species, Apicomplexa is distinguished by the prevalence of parasitic members.