Alternaria is a genus of ascomycete fungi. Alternaria species are known as major plant pathogens. They are also common allergens in humans, growing indoors and causing hay fever or hypersensitivity reactions that sometimes lead to asthma. They readily cause opportunistic infections in immunocompromised people such as AIDS patients. Not all Alternaria species are pests and pathogens; some have shown promise as biocontrol agents against invasive plant species. Some species have also been reported as endophytic microorganisms with highly bioactive metabolites.