Many antibiotics are derived from fungi. Penicillin is produced by Penicillium spp. Others change atmospheric nitrogen into forms available for plant use. Protozoa and Metazoa make up the majority of all parasites. They live whole or part of their lives on or within animals.
Life cycles are often complex with secondary hosts.
Protozoa are microscopic, single celled animals. One can give rise to 100's of millions in 24 hours. All are single celled. Animal's immune system repels with antibodies. Abbreviates subsequent reactions Lives and causes disease.