Last updated May 2023
Why is it that measles is said to be eliminated in the United States, yet there are still cases and outbreaks of it? What makes smallpox eradicated, and not just eliminated? In this article, we will discuss the terminology around infectious diseases used by scientists to describe the spread and impact of a disease. While there are other ways to eliminate or eradicate an infectious disease — like draining swamps and using mosquito repellents to eliminate malaria — vaccines have played a major role in controlling diseases that used to be common in many developed and developing countries.
Elimination of an infectious disease is declared when the disease is no longer found or transmitted at a sustained rate within a geographic area. For example, measles is no longer a “rite of passage” for children in the United States. There are still cases here and there, mostly imported by people who are not fully vaccinated. And there are outbreaks of the disease from time to time, mostly among unvaccinated or under-vaccinated populations. But measles is not seen at the rates as it was seen in the pre-vaccine era of the 1960s and earlier.
Other eliminated diseases in the United States include cholera, diphtheria, malaria, polio, rubella, and yellow fever. Diphtheria, polio, and rubella — like measles — were brought under control through the use of the MMR vaccine. Cholera was eliminated through water and sanitation, though the vaccine is still recommended and useful in places with sustained outbreaks, like Haiti in the Caribbean. Malaria and yellow fever were controlled through maintenance of breeding sites, mosquito repellents, and widespread availability of window screens and air conditioning.
Eliminated Diseases and Year of Elimination (Source of Information)
Cholera, 1978 ()
Malaria, 1951 ()
Measles, 2000 ()
Polio, 1979 ()
Rubella, 2004 ()
Yellow Fever, 1905 ()
Eradication of an infectious disease is declared when the disease is not found or transmitted anywhere in the world. The only human infectious disease to be eradicated through vaccination is smallpox. Polio is on the verge of being eradicated, with cases of the wild (not vaccine derived) strain only found in Central Asia. Rinderpest (a relative of measles that affected cattle) is the only animal disease to be eradicated through vaccination of cattle.
An infectious agent must fit several criteria to be eradicated. First, it must be easily detectable in the population. Smallpox caused a serious rash over the face and body, so people were identified and sent to isolation efficiently. People knew if they were exposed to a case, because they could see the disease when there was no laboratory test available, and those exposed were sent into quarantine. For those infectious diseases that do not have obvious physical manifestations, a highly precise laboratory test (with few false negatives) is needed. However, isolation and quarantine alone are not enough.
Second, the vaccine against the infectious agent must be highly effective and available to everyone at risk of contracting the disease. This guarantees most people getting the vaccine become immune to infection and/or disease, and that herd immunity is activated through vaccination of most of the population. For example, . That effectiveness drops for the whole population if a significant percentage of the population is not given the vaccine, triggering outbreaks.
Third, and closely associated with the second point, the public must accept the vaccine and get vaccinated. The vaccine could be highly effective, have an outstanding track record of safety, and be available at little to no cost to the entire population… But all that is meaningless toward eradication if enough people in the population will not get the vaccine. This is a big problem in the era of vaccine misinformation and disinformation spreading through online sources.
Finally, the infectious agent in question cannot be transmitted efficiently between animal species. For example, influenza is found in humans, birds, pigs, and other animals. In 2009, the H1N1 influenza pandemic happened when a strain jumped from pigs to humans and was able to be transmitted efficiently from person to person. Because of this, influenza is not a candidate for eradication, but it is a candidate for elimination if a universal influenza vaccine is developed.
There Is Hope, but There Are Challenges
The eradication of smallpox raised hopes that the same could be accomplished for other diseases, with many possibilities: polio, mumps, and dracunculiasis (), among others. Malaria has also been considered, and its incidence has been drastically reduced in many countries. It presents a challenge to the traditional idea of eradication, however, in that malaria does not lead to lifelong immunity against it (as smallpox and many other diseases do). It is possible to fall ill with malaria many times, although individuals may develop partial immunity after multiple attacks. Although promising steps have been made, there is no effective malaria vaccine available beyond clinical trials.
Other diseases present additional challenges. Polio, though it has been reduced or eliminated in most countries through widespread vaccination, still circulates in some areas because (among other reasons) many cases do not present easily recognizable symptoms. As a result, an infected person can remain unnoticed, but still spread the virus to others. Measles is problematic in a similar way: although the disease results in a highly visible rash, a significant period of time elapses between exposure to the virus and the development of the rash. Patients become contagious before the rash appears, and can spread the virus before anyone realizes they have the disease.
. Only 13 cases were reported in 2022, from the Central African Republic, Chad, Ethiopia, and South Sudan. The has declared six additional diseases potentially eradicable: lymphatic filariasis (Elephantiasis), polio, measles, mumps, rubella, and pork tapeworm.