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15.1 Characteristics of Infectious Disease

  • In an infection, a microorganism enters a host and begins to multiply. Some infections cause disease, which is any deviation from the normal function or structure of the host.
  • Signs of a disease are objective and are measured. Symptoms of a disease are subjective and are reported by the patient.
  • Diseases can either be noninfectious (due to genetics and environment) or infectious (due to pathogens). Some infectious diseases are communicable (transmissible between individuals) or contagious (easily transmissible between individuals); others are noncommunicable, but may be contracted via contact with environmental reservoirs or animals (zoonoses)
  • Nosocomial diseases are contracted in hospital settings, whereas iatrogenic disease are the direct result of a medical procedure
  • An acute disease is short in duration, whereas a chronic disease lasts for months or years. Latent diseases last for years, but are distinguished from chronic diseases by the lack of active replication during extended dormant periods.
  • The periods of disease include the incubation period, the prodromal period, the period of illness, the period of decline, and the period of convalescence. These periods are marked by changes in the number of infectious agents and the severity of signs and symptoms.

15.2 How Pathogens Cause Disease

  • Koch’s postulates are used to determine whether a particular microorganism is a pathogen. Molecular Koch’s postulates are used to determine what genes contribute to a pathogen’s ability to cause disease.
  • Virulence, the degree to which a pathogen can cause disease, can be quantified by calculating either the ID50 or LD50 of a pathogen on a given population.
  • Primary pathogens are capable of causing pathological changes associated with disease in a healthy individual, whereas opportunistic pathogens can only cause disease when the individual is compromised by a break in protective barriers or immunosuppression.
  • Infections and disease can be caused by pathogens in the environment or microbes in an individual’s resident microbiota.
  • Infections can be classified as local, focal, or systemic depending on the extent to which the pathogen spreads in the body.
  • A secondary infection can sometimes occur after the host’s defenses or normal microbiota are compromised by a primary infection or antibiotic treatment.
  • Pathogens enter the body through portals of entry and leave through portals of exit. The stages of pathogenesis include exposure, adhesion, invasion, infection, and transmission.

15.3 Virulence Factors of Bacterial and Viral Pathogens

  • Virulence factors contribute to a pathogen’s ability to cause disease.
  • Exoenzymes and toxins allow pathogens to invade host tissue and cause tissue damage. Exoenzymes are classified according to the macromolecule they target and exotoxins are classified based on their mechanism of action.
  • Bacterial toxins include endotoxin and exotoxins. Endotoxin is the lipid A component of the LPS of the gram-negative cell envelope. Exotoxins are proteins secreted mainly by gram-positive bacteria, but also are secreted by gram-negative bacteria.
  • Bacterial pathogens may evade the host immune response by producing capsules to avoid phagocytosis, surviving the intracellular environment of phagocytes, degrading antibodies, or through antigenic variation.
  • Viral pathogens use adhesins for initiating infections and antigenic variation to avoid immune defenses.
  • Influenza viruses use both antigenic drift and antigenic shift to avoid being recognized by the immune system.

15.4 Virulence Factors of Eukaryotic Pathogens

  • Fungal and parasitic pathogens use pathogenic mechanisms and virulence factors that are similar to those of bacterial pathogens
  • Fungi initiate infections through the interaction of adhesins with receptors on host cells. Some fungi produce toxins and exoenzymes involved in disease production and capsules that provide protection of phagocytosis.
  • Protozoa adhere to target cells through complex mechanisms and can cause cellular damage through release of cytopathic substances. Some protozoa avoid the immune system through antigenic variation and production of capsules.
  • Helminthic worms are able to avoid the immune system by coating their exteriors with glycan molecules that make them look like host cells or by suppressing the immune system.
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