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Molecular Virology and Microbiology

Houston, Texas

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Molecular Virology and Microbiology
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Infectious Diseases

 

Introduction to Infectious Diseases

Infectious diseases are responsible for a quarter to a third of all deaths worldwide. Children under the age of five are especially vulnerable, and infectious diseases account for over half of all deaths in this group. Of the top ten causes of death compiled by the World Health Organization for 2004, five were due to infectious diseases.

Top causes of death due to infectious disease worldwide

Ranking
Cause
Estimated number of deaths
(in millions)
Percent of all deaths

3

Lower respiratory infections

3.9

6.8

4

HIV/AIDS

2.8

4.9

7

Diarrheal diseases

1.8

3.2

8

Tuberculosis

1.6

2.7

9

Malaria

1.3

2.2

Source: The World Health Report, 2004, by the World Health Organization

 

The top single agent killers are HIV/AIDS, tuberculosis, and malaria. Lower respiratory infections (including pneumonia) and diarrheal diseases are caused by a variety of agents.

 

Agents that Cause Infectious Diseases

Image of rod-shaped bacteria Courtesy: CDC

Courtesy: CDC

Infectious diseases can be caused by several different classes of pathogenic organisms (commonly called germs). These are viruses, bacteria, protozoa, and fungi. Almost all of these organisms are microscopic in size and are often referred to as microbes or microorganisms. Although microbes can be agents of infection, most microbes do not infect humans or cause disease in humans.

The majority of agents that cause disease in humans are viruses or bacteria, although the parasite that causes malaria is a notable example of a protozoan.

Examples of diseases caused by viruses are HIV/AIDS, influenza, SARS, smallpox, diarrheal diseases, hepatitis, Ebola, and West Nile. Examples of diseases caused by bacteria are anthrax, tuberculosis, salmonella, respiratory and diarrheal diseases.

 

Transmission of Infectious Diseases

There are a number of different routes by which a person can become infected with an infectious agent. For some agents, humans must come in direct contact with a source of infection, such as contaminated food, water, fecal material, body fluids or animal products. With other agents, infection can be transmitted through the air.

The route of transmission of infectious agents is clearly an important factor in how quickly an infectious agent can spread through a population. An agent that can spread through the air has greater potential for infecting a larger number of individuals than an agent that is spread through direct contact. Another important factor is the survival time of the infectious agent in the environment. An agent that survives only a few seconds between hosts will not be able to infect as many people as an agent that can survive in the environment for hours, days, or even longer. These factors are important considerations when determining the risks of potential bioterrorism agents.

 

Impact of Infectious Diseases on Society

Infectious diseases have plagued humans throughout history, and in fact have even shaped history on some occasions. The plagues of biblical times, the Black Death of the Middle Ages, and the “Spanish flu” pandemic of 1918 are but a few examples. The 1918 flu pandemic killed more than a half million people in the United States and up to 50 million people worldwide and is thought to have played a contributing role in ending World War I.

Image of rod-shaped bacteria Courtesy: CDC

Courtesy: VOA

Epidemics and pandemics have always had major social and economic impacts on affected populations, but in our current interconnected world, the impacts are truly global. Consider the SARS outbreak of early 2003. This epidemic demonstrated that new infectious diseases are just a plane trip away, as the disease was spread rapidly to Canada, the United States, and Europe by air travelers. Even though the SARS outbreak was relatively short-lived and geographically contained, fear inspired by the epidemic led to travel restrictions and the closing of schools, stores, factories, and airports. The economic loss to Asian countries was estimated at $18 billion. A prolonged and more widespread outbreak would obviously have had a much greater economic impact.

The HIV/AIDS epidemic, particularly in sub-Saharan Africa, illustrates the economic and social impacts of a prolonged and widespread infection. The disproportionate loss of the most economically-productive individuals has reduced workforces and economic growth rates of affected countries, especially those with high infection rates. This impacts the health care, education, and political stability of these nations. In southern Africa where the infection rate is highest, life expectancy has plummeted in a mere decade from 62 years in 1990 -1995 to 48 years in 2000 – 2005. The existence of 12 million children under that age of 18 that were orphaned by HIV/AIDS in this region by 2003 highlights the impact of infectious diseases on families and societies.

 

Challenges in Infectious Disease Research

Despite significant advances in infectious disease research and treatment, the control and eradication of these diseases faces major challenges.  A WHO report released in 2007 warns that infectious diseases are spreading more rapidly than ever before and that new infectious diseases are being discovered at a higher rate than at any time in history.  In just the past five years, the WHO has identified over 1000 epidemics of infectious diseases including avian flu, polio, and cholera.  With greatly increased human mobility, infectious diseases have the potential to swiftly become global epidemics.  

Some of the reasons for the difficulty in combating infectious diseases are:

  • New infectious diseases continue to emerge
  • Old infectious diseases increase in incidence or geographical distribution
  • Old infectious diseases previously under control begin to re-emerge
  • The potential for intentional introduction of infectious agents by bioterrorists
  • The increasing resistance of pathogens to current antimicrobial drugs
  • Breakdowns in public health systems and communication between nations

These challenges are considered further in Emerging Infectious Diseases and Bioterrorism Agents.

 

For more information:

http://www.cdc.gov/node.do/id/0900f3ec8000e035 - information on infectious diseases from the Centers for Disease Control and Prevention (CDC)

http://www.niaid.nih.gov/dmid/ - - NIAID Division of Microbiology and Infectious Disease website

http://www.who.int/topics/infectious_diseases/en/ - information on infectious diseases from the World Health Organization (WHO)

 

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