Infectious Diseases
- Introduction to Infectious Diseases
- Agents that Cause Infectious Diseases
- Transmission of Infectious Diseases
- Impact of Infectious Diseases on Society
- Challenges in Infectious Disease Research
Introduction to Infectious Diseases
Infectious diseases are the second leading cause of death worldwide, after heart disease, and are responsible for more deaths annually than cancer. Infectious diseases claim 16.2 percent of people who die each year. Children under the age of five are especially vulnerable, and infectious diseases account for a disproportionate number deaths in this group.
Of the top ten causes of death reported by the World Health Organization in 2008, four were due to infectious diseases. However, low-income countries are more severely affected by infectious diseases, and in these countries, five of the top killers were due to infectious agents, with lower respiratory infections being the number one cause of death.
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 |
4.18 |
7.1 |
|
6 |
Diarrheal diseases |
2.16 |
3.7 |
|
7 |
HIV/AIDS |
2.04 |
2.5 |
|
8 |
Tuberculosis |
1.46 |
2.5 |
Source: World Health Organization, 2008
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
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.
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.
The most recent example is the H1N1 influenza or “swine” flu pandemic that began in the spring of 2009. For the first time in the long history of flu pandemics, we have been able detect the beginnings of an outbreak and monitor the spread of the disease on an almost daily basis as air travelers carried it around the globe. The new H1N1 flu traveled around the world with unprecedented speed and in a few short months made its impact felt globally. Even though the disease has been relatively mild for most people, some schools closed – more than 700 across the United States at the peak of the flu in May - and a number of infected people were quarantined. Mexico suffered great economic loss and damage to its tourism industry in an attempt to contain the outbreak in its early stage.
Officials are warning of the possibility of interruptions to schools and businesses as the flu season returns to the northern hemisphere in the fall. While there is no evidence that the virus is becoming more dangerous over time, significant disruptions could occur if the virus mutates to become resistant to antiviral drugs, such as Tamiflu, or if it begins to cause a more serious illness. A severe pandemic could deepen the current economic downturn, close schools and businesses for weeks, restrict social interactions, and lead to disagreements between nations regarding the allocation of limited doses of antiviral drugs and vaccine.
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, swine flu, polio, and cholera. With greatly increased human mobility, infectious diseases have the potential to swiftly become global epidemics and pandemics as evidenced by the current swine flu pandemic.
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
- Potential for intentional introduction of infectious agents by bioterrorists
- 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://www3.niaid.nih.gov/research/topics/ - Listing and information about infectious diseases from the National Institutes of Health (NIH)
http://www.cdc.gov/node.do/id/0900f3ec8000e035 - Listing and information about diseases, including infectious diseases, from the Centers for Disease Control and Prevention (CDC)
http://www.who.int/topics/infectious_diseases/en/ - Information about infectious diseases from the World Health Organization (WHO)
http://www.who.int/mediacentre/factsheets/fs310/en/index.html - Listing of the top ten causes of death compiled by the WHO
Learn more about:
Infectious Diseases Emerging Infectious Diseases Potential Bioterrorism Agents