Candida species cause infections in individuals with deficient immune systems. Th1-type cell-mediated immunity (CMI) is required for clearance of a fungal infection. Candida albicans is a kind of diploid yeast that commonly occurs among the human gut microflora. C. albicans is an opportunistic pathogen in humans. Abnormal over-growth of this fungus can occur, particularly in immunocompromised individuals. C. albicans has a parasexual cycle that appears to be stimulated by environmental stress.
The most common pathogenic species are Aspergillus fumigatus and Aspergillus flavus. Aspergillus flavus produces aflatoxin which is both a toxin and a carcinogen and which can potentially contaminate foods such as nuts. Aspergillus fumigatus and Aspergillus clavatus can cause allergic disease. Some Aspergillus species cause disease on grain crops, especially maize, and synthesize mycotoxins including aflatoxin. Aspergillosis is the group of diseases caused by Aspergillus. The symptoms include fever, cough, chest pain or breathlessness. Usually, only patients with weakened immune systems or with other lung conditions are susceptible.
The spores of Aspergillus fumigatus are ubiquitous in the atmosphere. A. fumigatus is an opportunistic pathogen. It can cause potentially lethal invasive infection in immunocompromised individuals. A. fumigatus has a fully functional sexual cycle that produces cleistothecia and ascospores.
Cryptococcus neoformans can cause a severe form of meningitis and meningo-encephalitis in patients with HIV infection and AIDS. The majority of Cryptococcus species live in the soil and do not cause disease in humans. Cryptococcus neoformans is the major human and animal pathogen. Cryptococcus laurentii and Cryptococcus albidus have been known to occasionally cause moderate-to-severe disease in human patients with compromised immunity. Cryptococcus gattii is endemic to tropical parts of the continent of Africa and Australia and can cause disease in non-immunocompromised people.
Infecting C. neoformans cells are usually phagocytosed by alveolar macrophages in the lung. The invading C. neoformans cells may be killed by the release of oxidative and nitrosative molecules by these macrophages. However some C. neoformans cells may survive within the macrophages. The ability of the pathogen to survive within the macrophages probably determines latency of the disease, dissemination and resistance to antifungal agents. In order to survive in the hostile intracellular environment of the macrophage, one of the responses of C. neoformans is to upregulate genes employed in responses to oxidative stress.
The haploid nuclei of C. neoformans can undergo nuclear fusion (karyogamy) to become diploid. These diploid nuclei may then undergo meiosis, including recombination, resulting in the formation of haploid basidiospores that are able to disperse. Meiosis may facilitate repair of C. neoformans DNA in response to macrophage challenge.
Histoplasma capsulatum can cause histoplasmosis in humans, dogs and cats. The fungus is most prevalent in the Americas, India and southeastern Asia. It is endemic in certain areas of the United States. Infection is usually due to inhaling contaminated air.