malaria (320x240)
credit: NIAIDColorized scanning electron micrograph of a red blood cell infected with malaria parasites, which are colorized in blue. In sub-Saharan Africa, malaria causes about 15% of deaths in young children.

Infectious diseases are disorders that are caused by organisms, usually microscopic in size, such as bacteria, viruses, fungi, or parasites that are passed, directly or indirectly, from one person to another. Humans can also become infected following exposure to an infected animal that harbors a pathogenic organism that is capable of infecting humans.

Infectious diseases are a leading cause of death worldwide, particularly in low income countries, especially in young children.

Three infectious diseases were ranked in the top ten causes of death worldwide in 2016 by the World Health Organization. They are lower respiratory infections (3.0 million deaths), diarrheal diseases (1.4 million deaths), and tuberculosis (1.3 million deaths). HIV/AIDS, which was previously on the list, has dropped from the global list of the top ten causes of death (1.0 million deaths in 2016 compared with 1.5 million in 2000), but it is still a leading cause of death in low income countries. Another infectious disease, malaria, accounts for a top cause of death in low income countries. 

Lower respiratory infections (including pneumonia) and diarrheal diseases are caused by a variety of infectious agents. The other infectious diseases on the list -  HIV/AIDS, tuberculosis, and malaria - are due to a single infectious agent. 

The Top Ten Leading Causes of Death



Low Income Countries

High Income Countries


Heart disease

Lower respiratory infections

Heart disease



Diarrheal diseases




Heart disease

Alzheimer's disease


Lower respiratory Infections


Lung cancers


Alzheimer's disease 




Lung cancers


Lower respiratory infections




Colon cancers


Road injury

Preterm birth complications



Diarrheal diseases

Birth asphyxia and trauma

Kidney diseases



Road injury

Breast cancer

Source:  World Health Organization (2018)

*COPD, Chronic obstructive pulmonary disease

Agents that Cause Infectious Diseases

cdc staph 1 (320x240)
credit: CDC - Janice Haney Carr Jeff Hageman, M.H.S.Scanning electron micrograph image depicting numerous clumps of methicillin-resistant Staphylococcus aureus bacteria; Magnified 9560x.

Infectious diseases can be caused by several different classes of pathogenic organisms (commonly called germs). These are virusesbacteria, 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 cause disease in humans. In fact, humans are inhabited by a collection of microbes, known as the microbiome, that plays important and beneficial roles in our bodies.

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/AIDSinfluenzaEbolaMERSsmallpoxdiarrheal diseases, hepatitis, and West Nile. Diseases caused by bacteria include anthraxtuberculosis, salmonella, and 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 in transmission 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 evaluating the risks of potential bioterrorism agents.

Impact of Infectious Diseases on Society

mers3b (320x240)
credit: NIAIDTransmission electron micrograph of Middle East Respiratory Syndrome Coronavirus particles, colorized in yellow.

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.

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 air travelers rapidly spread the disease to Canada, the United States, and Europe. 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, emerged in Saudi Arabia that causes Middle East respiratory syndrome, or MERS, a severe and often fatal respiratory illness. MERS has spread to other countries in the Middle East, as well as countries in Europe, Asia, and North America, including the United States. Infection occurs through direct contact with an infected animal (camel) or person, but if the virus were to adapt to humans over time, it could spread more easily from person to person.


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 approximately 18 million children worldwide under that age of 18 that have been orphaned by HIV/AIDS highlights the impact of infectious diseases on families and societies.


Another recent example of an infectious disease outbreak 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 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.

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

The sections on Emerging Infectious Diseases and Bioterrorism Agents further explore these challenges.