Research representing fungal pathogens and protozoa is also ongoing. One arena of investigation is the role that colonization of the gut by Candida albicans may have on the regulation of immune responses throughout the body. Another is research into the pathogenesis of the opportunistic fungi Cryptococcus neoformans and Aspergillis fumigatus and host defense against these microbes in the lungs and sinuses. Research on the common obligate intracellular protozoa Toxoplasma gondii is also being examined with respect to its molecular mechanism of entry into target cells of the central nervous system. This genetically and biochemically tractable parasite serves as an excellent model for invasion.