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Infectious diseases are illnesses caused by organisms that enter the body, develop, and multiply. These microorganisms can be bacteria, viruses, fungi, or parasites.  Infectious disease epidemiology focuses on the distribution, control of diseases as they relate to a particular population and tracing the causes of communicable diseases within a community.

Many infectious diseases are currently managed with the aid of modern medicine — new communicable diseases such as Covid19, present more challenges. Infectious disease epidemiology is also proving that older diseases, like tuberculosis and malaria, may now present in forms that are more resistant to established treatments. Professionals in the field of infectious disease epidemiology play a crucial role in managing the effects of both old and emerging illnesses.

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Epidemiologic analysis can be illustrated by a historical review of the human immunodeficiency virus (HIV) epidemic. A national epidemiologic surveillance case definition was developed after the acquired immunodeficiency syndrome (AIDS) was initially described in 1981. 

Disease surveillance was initiated to categorize the cases by standard measures of time, place, and person in an effort to identify population groups at risk.  Before the first laboratory evidence of an etiologic agent was presented, an infectious etiology was hypothesized early in the epidemic based on disease surveillance.  A combination of clinical epidemiologic and laboratory studies led to the identification of HIV as the cause of AIDS and to the development of sensitive and specific serologic tests for infection. In turn, this led to studies that characterized the spectrum of illness associated with HIV infection.  Epidemiologic studies of persons infected with HIV (with or without AIDS) indicated characterized routes of HIV transmission, depicted how other sexually transmitted infections increased the risk of HIV transmission and demonstrated that HIV infection could enhance the transmission of other agents, such as mycobacterium tuberculosis.  

Clinical trials were conducted to assess the efficacy of antiretroviral agents and combinations of drugs to increase the effectiveness of therapy and reduce the rate of resistance to individual drugs. The development of potential HIV vaccines progressed to the implementation and innovative design of phase III human trials.  Additional trials were conducted to assess the efficacy of a range of antimicrobial agents aimed at preventing a variety of opportunistic infections. Finally, using epidemiologic data, community-based programs were developed to promote behavior change aimed at reducing the risk of HIV transmission.   

In 2003, the global application of combined clinical, epidemiologic, and laboratory studies led to the rapid detection, characterization, and ultimately, control of an epidemic of severe acute respiratory syndrome (SARS) caused by a novel coronavirus.  Epidemiologic studies identified the original source of transmission from palm civets to humans through wild-animal markets in China and demonstrated the global spread of the epidemic through person-to-person transmission.  The eradication of the epidemic strain from humans and the identification of the wildlife reservoir of SARS coronaviruses established a framework for preventing future SARS outbreaks.


In 2012, the emergence of human infections caused by another novel coronavirus in the Middle East, the Middle East Respiratory Syndrome (MERS-CoV) again resulted in the combined rapid conduct of clinical, epidemiologic, and laboratory studies to reduce the risk of causing another outbreak and SARS-like epidemic.  

In 2013, public health authorities in China reported three cases of laboratory-confirmed human infection with a novel avian influenza A (H7N9) virus in an initial report. Two months after the initial report, laboratory-confirmed H7N9 infections reached 132, with 37 deaths. The rapid conduct of clinical, epidemiologic, and laboratory studies was initiated by Chinese medical and public health officials with support from experts from around the world.  Epidemiologic surveillance determined that four strains (H7 HA, N9 NA) of the H7N9 virus that evolved by genetic reassortment with avian influenza (H9N2) viruses, were circulating in the region. 

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The detection of the H7N9 virus in live poultry markets in the vicinity of human cases in eastern China, the contact history with live poultry markets in a substantial number of cases, and the major reduction in human cases after the closure of live poultry markets throughout eastern China, suggested exposure to live poultry as a key risk factor for human H7N9 infection.  Co-circulation of multiple avian influenza viruses that can infect humans highlights the need for increased and continued surveillance of poultry and potential environmental sources.  Epidemiologic methods have played a broad range of roles in understanding and controlling epidemics. 

When two different viruses co-infect the same host cell, they produce a new virus with a combination of both parental strains. When a virus from a host reassorts with a virus from another host it can create a potential pandemic virus. In this example, a human virus reassorted with an avian virus taking three of its segments. In particular, the segment coding for HA (indicated by a black arrow) in the resulting virus is of avian origin. 

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