Candida is a yeast liked fungus of gemmation, which has a wide distribution in the nature world. It can also be parasitize in normal human skin and the oral, gastrointestinal, anal and vaginal mucosa, with no occurrence of diseases. It is a typical conditional pathogen. Candida albicans is a common pathogen. Only when the immunity of the human body is compromised, Candida albicans can cause diseases. The pathogenesis of Candida includes three aspects: (1) The compromised cellular immunity of the human body. Its manifestations include candida antigen skin test negative. When stimulated in vitro by Candida antigen, the lymphocyte transformation rate is low and reduced/absent macrophage migration inhibitory factor synthesis. The amount of phagocytes is reduced, with loss of the chemotaxis and declining ability to swallow and kill the pathogens. (2) The pathogen. The spore wall of Candida albicans is mainly composed of glycogen and mannan which could strengthen the adhesion of Candida albicans to cause infection. (3) Iatrogenic factor. Iatrogenic factor includes the administrations of broad spectrum antibiotics, adrenal cortical hormone, and immunosuppressants, which provide chances for the invasion of the pathogens.

The histopathological changes include inflammatory (such as the skin, lungs), purulent (such as kidney, lung and brain) or granulomatous (such as skin). Special organs and tissues can also have special manifestations. For instances, ulceration occurs in the esophagus and small intestine; proliferative changes occur in the cardiac valves; micro-abscesses occur in the cases of acute disseminated diseases with spores and hyphae in the abscesses and external infiltration of the neutrophils and histocytes. Surrounding the spore, there occasionally are eosinophilic materials, like astral body. Sometimes, the hyphae invade into the vascular wall. The histopathological findings of hyphae have diagnostic value.

Candida erythema commonly occurs in the skin folds, such as the gluteal fold, groin, under the breast, axillary, umbilical fossa, as well as the labia. The manifestations include erythema erosion and dipping whitish membranous scales, with clear boundary, surrounding red macula, blisters or pustules, and sense of itches. It is common in patients with diabetes, overweight and hidrosis. Due to the immunodeficiency of AIDS patients, systemic skin candidiasis may occur, with manifestations of widespread erythema and scales lesions with clear boundary, scattered surrounding papules or vesicles. It is often accompanied by thrush or gastroenteritis.

Compromised immunity is an important factor inducing cryptococcal infection. Cryptococcal infection is rarely infected via skin contacts to Cryptococcus contaminated objects. Systemic cryptococcal disease with local manifestations is more common, which may be caused by spreading from meningeal, pulmonary or other lesions.

The basic pathological changes have two kinds: early diffuse infiltrative exudative changes, and later granuloma formation. In the early lesions, there are large quantity Cryptococcus neoformans gathering, encapsulated thallus by colloid capsule that avoids direct contacts between the thallus and the surrounding tissues. Therefore, the inflammatory responses of the tissue are not obvious. Granuloma formation occurs sever months after the infection, with proliferated giant cells, macrophages and fibroblasts, infiltration of lymphocytes and plasma cells, and occasional occurrence of necrosis and small cavities.

Skin Cryptococcus infection has manifestations of acne liked skin rash, papules, hard nodules, and granuloma. The lesions have central necrosis, ulceration and fistula. The mucosal lesions are commonly found in the oral cavity and nasopharynx, with manifestations of nodules, ulceration and granuloma, with coverage of the surfaces with adhesive exudative thin membrane.

Histoplasma is a biphasic fungus. It is in the yeast phase in the tissues and grows in the cells. By HE staining, round or oval globules can be found, with surrounding transparent halo like a capsule and a diameter of merely 2–4 um. PAS bacteria are slightly larger, with surround red stained cell wall to take the place of the capsule liked halo (actually the fungus has no capsule). It is highly contagious and can cause laboratory infection. The disease is caused by capsular histoplasmosis, which invades the human body via the skin. Its access into the human body causes primary or disseminated infections in people with compromised immunity.

The pathological basis is the non-specific inflammatory infiltration of primary histoplasmosis. Occasionally, the macrophages and necrotic area can be found. In the macrophages, there are large quantity pathogenic fungi; and histocytes with swallowed spores can also be found in the regional lymph nodes. In the early period, there is central proliferation, with more or less fungi in the macrophages. After that, tissue necrosis occurs, with surrounding granuloma changes and final fibrosis to heal.

Skin histoplasmosis has hard plaques, nodules and subcutaneous nodules of erythema liked changes.

Since the mid 1980s, leishmaniasis in HIV patients has been increasing dramatically. Leishmaniasis is a zoonotic disease caused by Leishmania and it transmits between arthropods and mammals, with occurrence in more than 80 countries. It has been estimated that the total number of infected patients is up to more than 15 million and the newly emerging cases are more than 0.4 million per year. For the HIV positive patients, leishmania may be a opportunistic pathogenic parasite.

The amastigote of leishmania can parasitize in macrophages and other phagocytic cells, such as monocytes and neutrophil. It causes chronic diffuse cells (including macrophages, lymphocytes and plasma cells) infiltration and the formation of micro granuloma with intact structure in patients with immunodeficiency.

Cutaneous leishmaniasis has two types: confined and diffuse. The manifestations include singular papula and nodule, with no occurrence of ulceration.

Skin toxoplasmosis is caused by toxoplasma gondii.

It is pathologically manifested as (1) inflammatory cell infiltration, and sometimes formation of granulomas to involve the blood vessels and skin appendages; (2) vascular changes of capillary dilation, endothelial cells proliferation, venules necrosis and perivasculitis; (3) squamous epithelial cells proliferation, with the demonstration of epitheliomatous symptom in some serious cases.

The manifestations include skin rashes, commonly in the face, trunk and limbs; and secondary scarlet fever liked desquamation. There are also high fever and general upset.

This disease caused by herpes simplex virus of DNA virus. Herpes simplex virus (HSV) can be divided into two types: HSV-1 and HSV-2. HSV-1 often causes the lips and corneal herpes while HSV-2 causes genital herpes which mainly spreads by direct contacts to the lesions (sexual contacts) to result in skin lesions. They can be primary and recurrent herpes.

Approximately 22 % cases of AIDS are complicated by herpes simplex. According to the diagnostic criteria by the CDC, chronic mucous herpes simplex or disseminated herpes simplex indicate the HIV infection has progressed in to AIDS.

Clinically, the lesions are erosions and ulcers with irregular margins, which is formed by rupture of intensive small blisters. Herpes simplex has atypical manifestations, with palm blisters or bullous lesions.