The development of an infectious disease is influenced by the patient’s general health, normal defense mechanisms, previous contact with the offending organism, past clinical history, and type and location of infected tissue. Mechanisms of host resistance are detailed in the following lists:

1. Primary host defenses

   1. Anatomic barriers

      1. Intact skin surfaces

      2. Nose hairs

      3. Respiratory tract cilia

      4. Coughing and flow of respiratory tract fluids and mucus

      5. Swallowing and gastrointestinal (GI) tract peristalsis

   2. Physiologic barriers

      1. High or low pH and oxygen tension (prevents proliferation of organisms)

      2. Chemical inhibitors to bacterial growth (e.g., proteases)

      3. Bile acids

      4. Active lysozymes in saliva and tears

      5. Fatty acids on skin surfaces

2. Secondary host defenses (physiologic barriers)

   1. Responses of complement, lysozymes, opsonins, and secretions

   2. Phagocytosis

   3. Immunoglobulin A, immunoglobulin G (IgG), and immunoglobulin M (IgM) antibody formation

   4. Cell-mediated immune responses

3. Factors decreasing host resistance

   1. Age (very young or very old)

   2. Presence of chronic disease (e.g., cancer, cardiovascular disease, diabetes)

   3. Use or history of certain therapeutic modalities, such as radiation, chemotherapy, corticosteroids, antibiotic drugs, or immunosuppressants

   4. Toxins, including alcohol; drugs (including legal, illegal, prescription, and nonprescription); venom or toxic secretions from a reptile or insect; or other nonhuman bites or punctures

   5. Others, including excessive physical or emotional stress states, nutritional state, and presence of foreign material at the site