Of the more than 100,000 species of fungi, only about 100 are pathogenic to humans and animals. On the other hand, thousands of fungal diseases affect economically important plants, costing in excess of one billion $$ annually. In general, healthy, immunocompetent individuals and animals have a high natural resistance to fungi. Infection and disease occur when there are disruptiong in the protective barriers of the skin and mucous membranes or when defects in the immune system allow fungi to penetrate, colonize, and reproduce in the host. Intact skin, fatty acid content, pH, epithelial turnover of the skin, and normal bacterial flora all contribute to host resistance.
Many Fungi that cause disease have developed mechanisms that enhance their survival and reproduction in the host. The surface components of the cell wall mediate attachment of fungus to host cells. Dermatophytes secrete keratinase, an enzyme that degrades keratin, a protein found in hair skin, and nails.
Many pathogenic fungi have tropism (directional movement responses that occur in response to a directional stimulus) for certain tissues, eg, dermatophytes thrive on keratin. A number of predisposing factors contribute to the establishment of fungal infections. They include:
1. prolonged administration of broad-spectrum antibiotics
2. exposure to a large number of spores.
Most fungi infections are generally chronic (long-lasting) infections because fungi grow slowly.
Unlike bacteria, fungi do not produce exotoxins or endotoxins.