Yeasts are unicellular eucaryotes . They reproduce by asexual budding . Most fungi are opportunistic pathogens since humans are usually resistant to infection unless they are immunocompromised or special conditions exist. Most Fungi don't like body temp, our skin pH, or low O2 Fungal infection can occur in people with unusual skin pH, smokers with damaged lung defenses, etc. Fungal infections do not commonly occur in people with normal immune function.
Systemic infections are the most serious and the most difficult to treat of the fungal infections.
Some fungi only exist as yeast. Thermally dimorphic fungi are molds at room temperature and are yeast at body temp. It means the fungal microbe can adjust to body conditions and thrive there. Fungal pneumonias are very serious. More so than bacterial or viral.