Chapter 113. Introduction to Infectious Diseases: Host–Pathogen Interactions (Part 1)

Harrisons Internal Medicine Chapter 113. Introduction to Infectious Diseases: Host–Pathogen InteractionsHost–Pathogen Interactions: IntroductionDespite decades of dramatic progress in their treatment and prevention, infectious diseases remain a major cause of death and debility and are responsible for worsening the living conditions of many millions of people around the world. Infections frequently challenge the physicians diagnostic skill and must be considered in the differential diagnoses of syndromes affecting every organ system.Changing Epidemiology of Infectious DiseasesWith the advent of antimicrobial agents, some medical leaders believed that infectious diseases would soon be eliminated and become of historic interest only. ...
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Chapter 113. Introduction to Infectious Diseases: Host–Pathogen Interactions (Part 1) Chapter 113. Introduction to Infectious Diseases: Host–Pathogen Interactions (Part 1) Harrisons Internal Medicine > Chapter 113. Introduction to InfectiousDiseases: Host–Pathogen Interactions Host–Pathogen Interactions: Introduction Despite decades of dramatic progress in their treatment and prevention,infectious diseases remain a major cause of death and debility and are responsiblefor worsening the living conditions of many millions of people around the world.Infections frequently challenge the physicians diagnostic skill and must beconsidered in the differential diagnoses of syndromes affecting every organsystem. Changing Epidemiology of Infectious Diseases With the advent of antimicrobial agents, some medical leaders believed thatinfectious diseases would soon be eliminated and become of historic interest only.Indeed, the hundreds of chemotherapeutic agents developed since World War II,most of which are potent and safe, include drugs effective not only against bacteriabut also against viruses, fungi, and parasites. Nevertheless, we now realize that aswe developed antimicrobial agents, microbes developed the ability to elude ourbest weapons and to counterattack with new survival strategies. Antibioticresistance occurs at an alarming rate among all classes of mammalian pathogens.Pneumococci resistant to penicillin and enterococci resistant to vancomycin havebecome commonplace. Even Staphylococcus aureus strains resistant tovancomycin have appeared. Such pathogens present real clinical problems inmanaging infections that were easily treatable just a few years ago. Diseases oncethought to have been nearly eradicated from the developed world—tuberculosis,cholera, and rheumatic fever, for example—have rebounded with renewedferocity. Newly discovered and emerging infectious agents appear to have beenbrought into contact with humans by changes in the environment and bymovements of human and animal populations. An example of the propensity forpathogens to escape from their usual niche is the alarming 1999 outbreak in NewYork of encephalitis due to West Nile virus, which had never previously beenisolated in the Americas. In 2003, severe acute respiratory syndrome (SARS) wasfirst recognized. This emerging clinical entity is caused by a novel coronavirusthat may have jumped from an animal niche to become a significant humanpathogen. By 2006, H5N1 avian influenza, having spread rapidly through poultryfarms in Asia and having caused deaths in exposed humans, had reached Europeand Africa, heightening fears of a new influenza pandemic. Many infectious agents have been discovered only in recent decades (Fig.113-1). Ebola virus, human metapneumovirus, Anaplasma phagocytophila (theagent of human granulocytotropic ehrlichiosis), and retroviruses such as HIVhumble us despite our deepening understanding of pathogenesis at the most basicmolecular level. Even in developed countries, infectious diseases have made aresurgence. Between 1980 and 1996, mortality from infectious diseases in theUnited States increased by 64% to levels not seen since the 1940s. Figure 113-1 Map of the world showing examples of geographic locales whereinfectious diseases were noted to have emerged or resurged. (Adapted fromAddressing Emerging Infectious Disease Threats: A Prevention Strategy for theUnited States, Department of Health and Human Services, Centers for DiseaseControl and Prevention, 1994.) The role of infectious agents in the etiology of diseases once believed to benoninfectious is increasingly recognized. For example, it is now widely acceptedthat Helicobacter pylori is the causative agent of peptic ulcer disease and perhapsof gastric malignancy. Human papillomavirus is likely to be the most importantcause of invasive cervical cancer. Human herpesvirus type 8 is believed to be thecause of most cases of Kaposis sarcoma. Epstein-Barr virus is a cause of certainlymphomas and may play a role in the genesis of Hodgkins disease. Thepossibility certainly exists that other diseases of unknown cause, such asrheumatoid arthritis, sarcoidosis, or inflammatory bowel disease, have infectiousetiologies. There is even evidence that atherosclerosis may have an infectiouscomponent. In contrast, there are data to suggest that decreased exposures topathogens in childhood may be contributing to an increase in the observed rates ofallergic diseases. Medical advances against infectious diseases have been hindered bychanges in patient populations. Immunocompromised hosts now constitute asignificant proportion of the seriously infected population. Physici ...