Microscopic, single-celled organisms that often cause recurrent diseases are called

this is known as Protozoan. Simply put it means any of a diverse group of eukaryotes, of the kingdom Protista, that are primarily unicellular, existing singly or aggregating into colonies, are usually nonphotosynthetic, and are often classified further into phyla according to their capacity for and means of motility, as by pseudopods, flagella, or cilia.