Infectious Diseases

Microbial Factors

The variety of microorganisms capable of infecting humans is broad and includes bacteria, viruses, fungi, protozoa, helminthes, arthropods, and, rarely, proteinaceous substances called prions. Organisms that invade or damage tissue in a healthy host are termed primary pathogens. Their virulence or capacity to cause disease depends on the number of organisms transmitted and their ability to enter tissues, evade the host's defenses and multiply, or produce extracellular products such as toxins. Organisms that invade and cause disease in a host with altered resistance are termed opportunistic pathogens. These organisms are often part of the host's normal flora, residing within the gastrointestinal or respiratory tracts, or may be acquired from the host's environment. They take advantage of the host's altered defenses, due to, for instance, genetic defects, immunosup-pressive therapy, cancer chemotherapy, or changes in the antimicrobial flora due to exposure to an antimicrobial drug.

Transmission

An exogenous disease, such as malaria, is caused by a microorganism whose natural environmental reservoir is outside the body. Other infections, such as appendicitis, are caused by a constituent of the indigenous microbial flora and designated as endogenous diseases. Organisms may be transmitted to the host by several different mechanisms including direct or indirect contact, which includes hand contact or a sneeze; contaminated food or water; contact with a contaminated inanimate object; or the bite of an insect vector. Entry thus may be by inhalation, ingestion, injection, or direct implantation.

Once transmitted, the organism colonizes the host's tissue at the site or portal of entry before undergoing a period of multiplication, leading to subsequent invasion of tissue and/or production of disease-causing toxins. An infectious disease becomes clinically evident when the microbe invades locally and/or disseminates throughout the body and produces tissue injury or organ dysfunction. Injury may be a direct effect of the organism or its toxins. In some cases, it may be due to the host's own inflammatory or immune response. Some organisms grow only at a specific body site, while others disseminate widely. The host and/or microbe determine the factors accounting for this tissue tropism. Some microbes are obligate intrac-ellular parasites and must invade cells for their survival; malaria spreads in this way. They often have surface molecules that facilitate entry into [Page 628]their target cells. Other microbes use the host's own scavenger cells but resist the normal killing mechanisms to survive, proliferate, and cause an infectious disease, such as tuberculosis.

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