Throughout the course of history, the landscape of human health has been shaped by infectious diseases: contagious conditions caused by viruses, bacteria, and other microscopic organisms. The black plague in the 14th century, the Spanish influenza in 1918, and the smallpox outbreak in the 18th century turned into wide-ranging epidemics. Modern epidemics such as SARS, the avian flu, and most recently, the 2014 Ebola outbreak in West Africa, further attest to the devastation that widespread infectious diseases can cause. From both past and present events, it’s clear that epidemics can profoundly affect human health and culture.

But given the diversity of infectious diseases, is it possible to determine the most prevalent and those having the greatest impact on human health? This is a far more complicated question than it appears. While our perception of infectious diseases is often shaped by their popularity in mainstream media and culture, the prevalence of a disease in the press or public imagination is not always reflective of its incidence in the world. Indeed, given that mainstream reporting tends to focus on rare, dramatic diseases, the media coverage can distort our perception of which diseases are truly relevant threats. To further complicate the issue, diseases affect communities differently from how they are reported to how they are dealt with by local medical and health organizations.

When it comes to diseases, there is no one-size-fits-all definition of “most prevalent.” We can assess the effect of a disease from many angles: a disease that we might not consider to be most influential based on one metric may in fact have greater impacts on other aspects of health.

Read on to learn more about several diseases that are among most prevalent and have the most significant impacts worldwide.

Human Immunodeficiency Virus

Since it was recognized as a disease in the 1980s, acquired immunodeficiency syndrome (AIDS), caused by the human immunodeficiency virus (HIV), has had a significant impact on global health, society, and culture. The WHO lists HIV, an incurable, fatal condition, among the top 10 leading causes of deaths: the virus kills over 1.5 million people and infects 2.1 million new people per year. It is the only infectious disease to make WHO’s top 10 list, making it arguably the most prevalent infectious disease in terms of deaths. While HIV is known primarily as a sexually transmitted infection (STI), it can be transmitted through contact with infected blood. Such contact can arise from blood transfusions, maternal transmission, and needle sharing.

Despite its indelible mark on human history, HIV is a relatively recent phenomenon—the virus is thought to have first originated in the 1920s, and it gained pandemic status in the early 80s. Understanding of the disease in those early days was hampered by skepticism towards the idea that infection by a single virus could elicit the wide spectrum of symptoms associated with AIDS. But by 1983, American and French scientists had reported that a virus—later to be called HIV—was indeed the source of AIDS: they were awarded the prestigious Nobel Prize in Medicine and Physiology for their game-changing research in 2008. The revolutionary discovery of the virus and the subsequent flood of efforts to understand it have swiftly paved the way to effective treatments, such as anti-retroviral therapy, and it has demonstrated the ability of science and medicine to rally rapidly and effectively, even against a totally novel, widespread global threat.

Tuberculosis (TB)

Tuberculosis (TB), a disease of the lungs caused by the bacteria Mycobacterium tuberculosis, is the second leading cause of death by infectious agents, after HIV/AIDS (in fact, HIV-positive individuals are 26 to 31 times more likely to develop active TB). It is estimated that about one third of the world’s population carries latent TB, which has the potential to later develop into active, contagious illness. Ultimately, 9 million people are estimated to fall ill with active TB every year, leading to about 1.5 million deaths, largely in lower- and middle-income countries.

Though TB is treatable, concerns have developed over the emergence of strains of multi-drug resistant TB (MDR-TB), which does not respond to the two most common TB treatments. Worldwide, MDR-TB is estimated to account for 500,000 of TB cases. Some MDR-TB strains can develop even greater resistance to additional classes of drugs; such extensively drug-resistant TB (XDR-TB) requires far greater resources to treat, and unfortunately, many nations and regions lack such infrastructures. Ongoing efforts by the WHO emphasize the need to control MDR-TB and XDR-TB, both by improving treatments and by improving patient access to these resources.

Fortunately, despite problems with drug resistance, TB cases are declining worldwide. In fact, the world may be on track to achieving the UN’s Millenium Development Goal of halting and reversing the spread of TB by 2015. Still, the fight against TB is an ongoing, slow process, and the WHO has called for new “global solidarity and action” to support a new, ambitious goal to end the TB epidemic by 2035.

Chlamydia

Chlamydia is the disease caused by the bacterium Chlamydia trachomatis. Best known as a disease of the genital system, chlamydia is transmitted sexually, affects men and women, and in some cases—surprisingly—causes irreversible blindness. There are estimated to be 105.7 million new cases a year, about 50-fold the number of new annual HIV cases.

If not properly treated, chlamydia can lead to crippling or even fatal complications. In women, untreated chlamydia increase the risk of ectopic pregnancies—a life-threatening condition. Furthermore, infection of C. trachomatis in the eye is the world’s leading cause of preventable blindness. This condition, called trachoma, is responsible for the irreversible blindness of 1.2 million people worldwide, particularly in poorer, rural areas. Thus, even though chlamydia in and of itself is not fatal, untreated chlamydia can facilitate debilitating and fatal conditions, particularly in low- and middle-income countries, which may lack the screening and management programs necessary for timely and thorough treatment.

Table of statistics

Disease US Worldwide
HIV/AIDS 13,712 deaths, 1.2 million cases total (50,000 new cases). Source: CDC 2012 1.5 [1.4–1.7] million deaths. 35.0 [33.2–37.2] million cases (2.1 [1.9–2.4] million of these were new infections). Source: WHO 2013
Chlamydia 1.4 million new cases (NB: there is no obvious statistic for “deaths” since chlaymdia in and of itself is not fatal, but can subsequently lead to fatal complications). Source: CDC 2013 105.7 million new cases. Source: WHO 2008
Gonorrhea 333,004 new cases. Source: CDC 2013 106.1 million new cases. Source: WHO 2008
HPV 14 million new people (estimated – there is no routine “HPV test” as testing is only done in high-risk cases). Source: CDC 2013.

Cervical cancer: 12,109 new diagnoses

4,092 deaths

Source: CDC 2011

50-80% of the world’s population is affected at one point in their lives. Source: Nobelprize.org

Cervical cancer: 529,000 new cases, 266,000 deaths

Source: WHO 2008

Hepatitis B 18, 760 new cases (estimated; reported number is much lower)Source: CDC 2012 240 million chronic cases, 780 000 deaths (various causes: cirrhosis, cancer, acute infection). Source: WHO 2012
TB 9,582 new cases. Source: CDC 2013 9 million new cases, 1.5 million deaths. Source: WHO 2013
Malaria eliminated in US, all cases are from recent travelers. The high in recent years was 1,925 in 2011. 198 million new cases, 584,000 deaths. Source: WHO 2013

Gonorrhea

Gonorrhea is another bacterial STI, caused by Neisseria gonorrheae. Similar to chlamydia, it is very common. There are an estimated 106.1 million new cases of gonorrhea annually worldwide, and like chlamydia, it can also increase risk for ectopic pregnancies.

Infections by N. gonorrheae were once easily treatable with antibiotics. However, over the last 50 years, medical treatments for gonorrhea have become, alarmingly, less effective. As with TB, different strains of N. gonorrheae have rapidly evolved to become resistant to many existing antibiotic treatments, with the sole exception of cephalosporins. Even then, there are reports that some gonorrhea strains are also becoming cephalosporin resistant. Ongoing efforts by scientists and pharmaceutical companies to improve treatment are being outsmarted by the bacteria, leading to fears that untreatable cases of this disease may soon become a worldwide problem.

Human Papilloma Virus (HPV)

The human papilloma virus (HPV) is an extremely common infection: it is estimated that 50-80% of the world’s population is infected by HPV at least once in their lifetime. Thus, HPV can be considered the most prevalent STI worldwide based on the sheer number of people who are affected. More precise estimates of HPV cases are unavailable, since most HPV cases go unreported,and the test for symptoms of HPV, known as the Pap smear, is only performed in women (there is no routine test for men). Many HPV cases also go unreported due to the nature of the virus: of the over 40 types of sexually transmitted HPV, many strains are harmless and even harmful strains can be cleared by an individual’s immune system without need for intervention.

But while HPV infection can be harmless and short-lived, the virus has dire consequences when it sticks around. HPV was found to be linked to cervical cancer, a discovery that was awarded the Nobel Prize in Medicine and Physiology in 2008, concurrent with the HIV/AIDS discovery. HPV is responsible for over 5% of cancers worldwide and is confirmed in 99.7% cases of cervical cancer in women. Globally, cervical cancer is the second most common cancer in women, and is a particularly heavy burden in developing countries. 85% of cervical cancer cases occur in developing countries, and within these countries, it accounts for 13% of all female cancers. Other, rarer cancers can sometimes be caused by HPV (e.g.: anal, vaginal, vulvar, penile, and oral cancers). Milder cases of HPV infection causes genital warts in both sexes.

Gardasil, a vaccine against HPV, was developed by the pharmaceutical company Merck and approved by the United States government in 2008. Marketing campaigns by Merck aimed at parents stated that with Gardasil, “Your daughter could be one less life affected by cervical cancer.” However, this strategy has been criticized for failing to acknowledge that while Gardasil is effective against about 70% of cancer-associated HPVs, it does not protect against all types. The FDA has just approved an updated vaccine, Gardasil 9, which protects against an additional 20% of cancer-associated HPVs, bringing the total protection up to 90%.

Although Gardasil is primarily advertised towards women, it is also effective in men. Though risk of HPV-caused cancers in males, specifically anal, oral, and penile cancers, is very low, all men are still recommended to receive the vaccine to prevent genital warts and transmission of cancer-causing HPV to women. Men who have sex with men are also recommended to get the HPV vaccine.

Hepatitis B

Hepatitis B is a liver disease caused by the Hepatitis B virus (HBV) and is spread through contact with infected bodily fluids. While Hepatitis B is commonly spread at birth or in early childhood, particularly in Sub-Saharan Africa and East Asia, it can also be spread through sexual contact. It is estimated that about 2 billion people—just a little under a third of the world’s population—have been infected with Hepatitis B. More than 240 million people suffer from chronic HBV infections, with 780,000 associated deaths a year.

Despite the prevalence of HBV, Hepatitis B can be very effectively prevented with the HBV vaccine. First developed in 1982, the vaccine is 95% effective against HBV-related complications, including liver cancer. It was considered to be the first vaccine against a major cancer.

Malaria

Malaria, caused by the Plasmodium falciparum parasite that is carried by mosquitos, is a very prevalent disease thought to cause 198 million new cases a year. It causes about 584,000 deaths, most of which occur among sub-Saharan African children: it is estimated that one child dies from malaria in Africa every minute.

Malaria is curable, though there are many recurring challenges to controlling the disease such as the challenge of developing effective surveillance programs and the resistance of the Plasmodium parasite to medications. Currently, the most effective malaria interventions focus on prevention of exposures to mosquitos that carry the infectious agent. Such measures include use of mosquito nets and spraying with insecticides. Despite ongoing challenges, there are hopes of malaria eradication: the Bill and Melinda Gates Foundation has championed these goals. Researchers, though, have expressed hesitation towards current abilities to realize this ambitious endeavor, and they have urged continual research into more effective measures of eradication.

So which of these seven diseases is the world’s “most prevalent?” Ultimately, the answer is “it depends.” One could argue, for instance, that HIV or TB are the most prevalent because they cause the most deaths. However, other diseases could be considered more prevalent because they affect a much larger number of individuals. Though these diseases may not cause as many fatalities, they can nonetheless drastically decrease the quality of life of infected individuals. For instance, chlamydia itself is not fatal, but can lead to blindness, a severe and life-changing condition. Thus, though number of deaths is one metric, it does not necessarily capture other burdens that are more difficult to document.

Additional issues arise when one considers the variable natures of infectious diseases. For instance, there are 9 million cases of TB illness annually—lower than the number for any other disease on this list. However, there are over 2 billion people with latent TB, who have potential to develop the disease later. A similar situation can be observed for Hepatitis B: though it causes fewer deaths than HIV or TB, it still infects over 2 billion people worldwide.

Difficulties in screening and detecting diseases present additional challenges. HPV is extremely common and may infect up to 80% of all individuals worldwide—but the lack of simple and routine screening means its true burden remains unclear. Similarly, we can only estimate the true reach of malaria, as surveillance programs are thought to capture only a small fraction of all cases.

Moreover, the recurring difficulties of managing TB, gonorrhea, and malaria due to their ability to evolve to resist treatment reflects the rising challenge of global antimicrobial resistance. The WHO considers antimicrobial resistance a serious threat to public health in all parts of the world. There are fears that without proper action, other diseases—some that didn’t make it on this short list—will become untreatable and far more difficult to contain.

In short, the main take-away should be that infectious disease are very common and often spread faster and farther than they can detected and subsequently treated. Very often, the most threatening diseases can remain latent for a period of time. Nonetheless, with increased surveillance, awareness, and preventative efforts, it’s possible to decrease their public health burden for affected individuals worldwide.


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Image Credit: NIAID from Flicker

About The Author

Vivian Chou

Vivian Chou is a Ph.D. student in the Biological and Biomedical Sciences Program at Harvard Medical School.