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Molecular Virology and Microbiology

Houston, Texas

BCM has 25 departments and more than 90 research and patient-care centers.
Department of Molecular Virology and Microbiology
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Infectious Diseases

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

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


Lower respiratory infections




Diarrheal diseases











Source: World Health Organization, 2008 updated June 2011

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

Scanning electron micrograph of Staphylococcus aureus bacteria (gold) outside a white blood cell (blue). Credit: NIAID
Colorized scanning electron micrograph of red blood cell infected with malaria parasites, which are colorized in blue. The infected cell is in the center of the image area. To the left are uninfected cells with a smooth red surface. Credit: NIAID
Transmission electron micrograph of Middle East Respiratory Syndrome Coronavirus particles, colorized in yellow. Credit: NIAID

Credits: NIAID

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 Chinese students wearing masks during the SARS epidemic.

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. Recently, a new SARS-like virus, named MERS-CoV, has emerged in the Middle East, which causes a severe respiratory illness that has been fatal to approximately half of the people infected with this virus. It is currently too soon to know if this virus will continue to spread throughout the Middle East and beyond.

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.

Another 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, the beginnings of an outbreak were detected and the spread of the disease was monitored 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 was relatively mild for most people, some schools closed – more than 700 across the United States at its peak - 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. After the H1N1 flu waned, yet another variant of influenza virus has emerged. The new H7N9 virus was first detected in birds and humans in China in the spring of 2013, but it currently does not appear to be spreading from person to person.

A severe pandemic potentially could cause major disruptions to national and global economies, 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: - Listing and information about infectious diseases from the National Institutes of Health (NIH) - Listing and information about diseases, including infectious diseases, from the Centers for Disease Control and Prevention (CDC) - Information about infectious diseases from the World Health Organization (WHO) - Listing of the top ten causes of death compiled by the WHO

Learn more about:

Infectious Diseases Emerging Infectious Diseases Potential Bioterrorism Agents

Anthrax Ebola HIV/AIDS HTLV Influenza MRSA Dengue

Norovirus SARS Smallpox Tuberculosis Tularemia