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Emerging and Re-Emerging Microbes

With the arrival of anibiotics and modern vaccines, as well as improved sanitation and hygiene, many diseases that formerly posed an urgent threat to public health were brought under control or largely eliminated. As a result, by the mid-20th century some scientists thought that medical science had conquered infectious diseases.

Despite these public health advances, new microbes emerge and old microbes re-emerge just as they have throughout history. Several pressures contribute to the emergence of diseases, such as

  • Redistribution of human populations
  • Rapid global travel
  • Changes in the way we use land
  • Ecological, environmental, and technological changes

Practices such as the misuse of antibiotic medicines also contribute to disease emergence.

In addition, unsanitary conditions in animal agriculture and increasing commerce in exotic animals (for food and as pets) have contributed to the rise in opportunity for animal microbes to jump from animals to humans. From time to time, with the right combination of selective pressures, a formerly harmless microbe found in humans or animals can evolve into a pathogen that can cause a major outbreak of human disease. These pressures are shaping the evolution of microbes and bringing people into closer and more frequent contact with pathogens.

Emerging Microbes

Scientists usually define emerging microbes as those that have appeared only recently in a population or have existed but are rapidly increasing in incidence or geographic range. Recent examples of such disease-causing microbes are methicillin-resistant Staphylococcus aureus (MRSA) bacteria, West Nile virus, and 2009 H1N1 influenza virus.

photo of a fruit bat
Fruit bats in India and Malaysia have been found to carry Nipah virus.

Some newly emeriging/emerged microbes

  • 2009 H1N1 influenza virus
  • Ebola virus
  • H5N1 avian influenza virus
  • Marburg virus
  • MRSA bacteria
  • Multidrug-resistant and extensively drug-resistant tuberculosis bacteria
  • Nipah virus
  • SARS virus
  • West Nile virus


Re-Emerging Microbes

The reappearance of microbes that had been conquered successfully or controlled by medicines and vaccines is distressing to the scientific and medical communities, as well as to the public. One major cause of disease re-emergence is that many microbes responsible for causing these diseases are becoming resistant to the drugs used to treat them. Some examples of re-emerging infectious diseases that are of significant public health concern are dengue, malaria, TB, and polio.

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Last Updated July 25, 2014

Last Reviewed July 25, 2014