Mycotic infectious agents can be either true pathogens or opportunists. Mycotic infections cause allergies, toxicoses, and specific disease conditions. Many mycotic infections are seasonally related. Virulence factors include resistant spores, thermal dimorphism, toxin production, and invasive factors. The infective agents are usually spores, which are widely distributed through dust and air.

Fungal infections are categorized by their pathogenicity and the level or type of infection they cause. They are grouped by site of infection: cutaneous, subcutaneous, and systemic. True pathogens are species which are capable of initiating infection in a healthy host. Such pathogens exhibit thermal dimorphism, exhibiting the yeast form at body temperatures and the mold form at lower temperatures. True pathogens are Histoplasma, Blastomyces, Coccidioides, and Paracoccidioides.

Opportunists have few if any virulence factors, may or may not be thermal dimorphs, and initiate infection only in immunodeficient hosts. Examples are Cryptococcus, Candida, and Aspergillus. Some fungi are not inherently pathogenic but, if introduced subcutaneously, have the potential to cause serious infection. Examples are the dermatophytes and Sporothrix species.

Mycoses are not usually communicable, with certain exceptions. The four pathogenic species are endemic to specific ecological regions and exhibit seasonal patterns of infection. Epidemics occur because of mass exposure to a common source.

All true pathogens are dimorphic. They all initiate infection through the respiratory route. All elements of the fungus can initiate infection, but spores are the most common. The yeast phase is the more invasive form. Host defenses to fungal infection depend on the integrity of all epithelial barriers, a functional inflammatory response, and cell-mediated immunity. Effective treatment of fungal infections requires rapid and accurate diagnosis because antibiotic therapy can vary with the species.

Accurate diagnosis of mycotic agents requires direct microscopic examination of fresh specimens followed by confirmatory isolation on solid media and serological tests for host antibody.

Immunization is not usually effective against fungal infections. Preventives are limited to masks and protective clothing to reduce contact with spores. Treatment includes antifungal therapy and, in some cases, surgery.

Systemic Infections by True Pathogens
Primary fungal pathogens have several common characteristics: 1. Each is endemic to a specific region of the world. 2. All cause primary pulmonary infections by inhalation of spores. 3. In most cases, the infection is not life-threatening. 4. Recovery from infection confers lifelong immunity. True fungal pathogens cause systemic infection in certain susceptible groups of people. The spores of some species can also infect the skin.

Histoplasma capsulatum is the causative agent of histoplasmosis, or Ohio Valley fever. It is endemic to the Ohio Valley in the United States and discontinuously distributed on all other continents except Australia. Airborne spores cause pulmonary, systemic, or cutaneous lesions. The severity of the infection depends on the number of spores inhaled and the immunocompetence of the infected host. H.capsulatum is identified by ``fish-eye'' yeast cells in host macrophages, complement fixation tests, and host antibody titer.

Coccidioides immitis is the causative agent of coccidiomycosis, or Valley fever. It is endemic to salty soils in arid regions of the Western Hemisphere. Coccidioidomycosis is most prevalent in summer and fall. Airborne spores cause a primary pulmonary infection which is self-limiting in most cases and results in a lifelong immunity. Rare cases progress to chronic pulmonary disease or systemic infections. C.immitis is identified by highly distinctive spherules in fresh tissue specimens.

Blastomyces dermatitidis is the causative agent of blastomycosis, or Gilchrist's Disease. It is endemic to the Western Hemisphere, Africa, and the Middle East. Like other true pathogens, airborne spores cause a primary pulmonary infection, but the skin is also a site of infection. Chronic blastomycosis progresses systematically, producing abscesses in the skin and other organs. B.dermatitidis is identified by its microscopic appearance in fresh specimens and by serological tests for antibody.

Paracoccidioides brasiliensis is the causative agent of paracoccidioidomycosis, or South American blastomycosis. It is endemic to regions of Central and South America. Like B.dermatitidis, its spores infect both the lungs and the skin. In most cases, these infections are self-limiting. Chronic infections involve the lungs, skin, mucous membranes and the lymphatic system. P. brasiliensis is identified by its unusual budding pattern and by serological tests for antibody.

Subcutaneous Mycoses
Fungal infections that invade traumatized skin are called subcutaneous mycoses. These localized infections rarely become systemic, but they can be very destructive to the skin and its associated components. Sporotrichosis, chromoblastosis, phaeohyphomycosis, and mycetoma are examples.

Sporothrix schenkii is the causative agent of sporothrichosis, or rose-gardener's disease. When introduced subcutaneously, it produces local lesions with the potential to invade lymphatics.

Chromoblastomycosis and Phaehyphomycosis are both caused by certain pigmented soil saprobes that produce characteristic slow-growing skin lesions of low virulence.

Mycetoma, or madura foot, is caused by filamentous fungi that invade traumatized skin. Lesions appear as abscesses. The systemic form spreads to bones and muscles, and is very difficult to treat.

Cutaneous Mycoses
Dermatophytoses are fungal infections of the nonliving epidermis and its derivatives. The causative agents are members of the genera Trichophyton, Microsporum, and Epidermiphyton. Dermatomycoses are classified according to the body region infected. They are also called ringworm or tinea infections. All three dermatophyte genera are communicable. Microsporum is also transmitted from soil to humans.

Superficial Mycoses
Superficial mycoses are non-invasive, noninflammatory fungal infections restricted to the hair and outer layer of the epidermis. Superficial mycoses of the hair, or piedra, appear as white or black masses on individual hair shafts. Epidermal infections include tinea versicolor, folliculitis, psoriasis, and seborrheic dermatitis.

Opportunistic Mycoses
Candida albicans is the causative agent of a wide variety of opportunist infections. These range from minor skin mycoses to fatal systemic diseases. Candida infections are most likely to occur in hosts with lowered resistance, trauma, invasive devices, or those receiving prolonged antibiotic therapy for bacterial infections.

Cryptococcus neoformans is the causative agent of cryptococcosis, a non-communicable mycosis which infects the respiratory, mucocutaneous, and nervous systems. Cryptococcosis is commonly found in bird droppings and is spread through air and dust. Those at greatest risk are people who work in pigeon-infested areas, AIDS patients, and other immunocompromised hosts. Infections are self-limiting in healthy hosts, but invasive in susceptible hosts.

Aspergillosis is a fungal infection caused by the spores of Aspergillus species. Particularly high concentrations of spores occur in dust from granaries, barns, and silos. Inhaled spores germinate in the lung to form ``fungus balls,'' which remain localized in healthy hosts. Susceptible hosts may develop systemic infections which have a very poor prognosis.

Pneumocystis carinii is an opportunistic fungus of the upper respiratory tract that causes pneumonia in AIDS patients and other immunosuppressed hosts. Originally classified as a protozoan, it is not known to be related to the yeast Saccharomyces.

Mucormycosis is caused by saprobic fungi which colonize debilitated hosts in acidosis. Mucormycosis invades through several portals of entry and progresses rapidly to a systemic infection if not treated early.

Species in the genera Geotrichum and Fusarium are the causative agents of geotrichosis, a rare mycosis which infects the lungs of susceptible hosts. Geotrichosis also infects the nail bed, eyes, and burned skin.

Fungal Allergies and Intoxications
In addition to agents of disease, fungi act as allergens and toxins. Many of the allergies are related to agricultural occupations, but some are also common among the general population. Claviceps and Aspergillus species produce toxins that are hallucinogenic, hepatotoxic, or carcinogenic.