Did you know that smallpox is the only infectious disease to be considered officially eradicated by the World Health Organisation (WHO)? And that one person with measles can infect 12-18 people, making it the most infectious disease? Both smallpox and measles are two very contagious viral diseases that display very similar symptoms despite being caused by two different viruses. Both illnesses have caused a lot of fatalities throughout human history, but neither is now among the leading causes of death because of successful widespread vaccination campaigns.
The definition of smallpox
Smallpox is an ancient and feared viral infectious disease caused by the variola virus. Smallpox is thought to have existed for 3000 years before it was eradicated by a very successful WHO smallpox immunisation program that started in the 1960s.
Historically, smallpox has been extremely lethal, ravaging the human population and killing millions, with a considerable fatality rate of 12-30%.
Smallpox is an eradicated viral infectious illness. The very contagious variola virus causes smallpox.
The last reported smallpox natural case happened in 1977 in Somalia. After three years without any natural cases, this infectious disease was considered the first and only time in human history to have been eradicated by the WHO in 1980. This achievement remains one of the most relevant public health success stories in medical history.
After eradicating smallpox in the last century, only two places on earth are known to store the smallpox variola virus. The Centre for Disease Control (CDC) in the US and the Russian State Centre for Research on Virology and Biotechnology (VECTOR institute) in Russia are the only high-security lab facilities designated by the WHO as storing the highly contagious virus. Smallpox outbreak concerns are now exclusively related to the possibility of bio-terrorists using this viral infectious agent in biological warfare rather than naturally occurring smallpox epidemics.
The definition of measles
Measles, like smallpox, is a highly contagious human viral infectious disease. A virus of the paramyxovirus family causes measles. Despite not being a leading cause of death anymore like other more deadly infectious diseases such as HIV, malaria or tuberculosis (TB), measles still kills many people a year. According to 2018 WHO statistics, more than 140 000 people died of this illness that year. Millions were infected, particularly where measles remains a major disease.
However, unlike smallpox, measles has not been eradicated and remains in the community.
Measles is a viral infectious disease caused by a virus of the paramyxovirus family. Measles is one of the most contagious diseases and a leading cause of infant mortality in countries with high incidence.
Measles fatalities have been reduced significantly in the last decades due to the development of a safe and cost-effective vaccine and its widespread immunisation effort throughout the world. The WHO data shows a 73% drop in measles deaths due to vaccinations in this century, avoiding an estimated 20 million deaths.
Any non-immune person can contract measles. Immunisation can either result from vaccination through the measles vaccine or through natural exposure to the causative measles viral agent. Since there is only one strain of the measles virus, it’s very unlikely that someone will catch this disease twice because natural immunity will protect them the second time from developing a measles infection.
Very young children (a few months old) have passive immunity because of maternal antibodies that were transferred via the placenta during the pregnancy. However, this is a short-lived immunity as maternal antibody levels quickly deplete after a few months.
The causes of smallpox and measles
In most instances, smallpox disease is caused by inhaling the variola virus. This infectious disease is usually transmitted through direct personal contact via infective respiratory droplets released in a contagious person’s breath while talking, coughing or sneezing. Smallpox can also be transmitted through contaminated bedding or clothing in less common instances. After exposure, the variola virus incubates for 7 to 17 days before a person becomes infectious.
Fig. 1: Smallpox and measles transmission.
The beginning of the infective stage of smallpox is marked by the development of a fever followed by a typical rash two to three days after. This initial first week of illness period is often the most infectious. However, infected smallpox patients primarily infect their household members and not the general public. This is because contagious people are already sick and most likely on bed rest and semi-isolated from the overall community. In most cases, people with smallpox infection recovered, but up to 30% of the time, they passed away due to systemic shock and toxinemia (toxins in the blood).
Systemic shock is a fatal condition whereby infection triggers body-wide inflammation and dangerously low blood pressure.
This results from the patient’s inability to regulate their immune response to infection.
As for measles, this infectious disease is also caused by breathing in virus droplets. The viral infectious particles are released through coughing or sneezing of an infected individual and can remain in suspension in the air and on surfaces for 2 hours. Once it enters the body, the measles virus targets cells of the respiratory tract, starting its incubation period of up to 14 days with no obvious visible symptoms. Shortly before symptom onset, an infected individual starts to become contagious. On most occasions, the measles illness fades out after several days without complications. The appearance of complications is associated with severe measles, which can often lead to death, especially in certain risk groups.
The incubation period describes the time taken for clinical manifestations of infection to appear after the initial exposure to a pathogen.
Deaths caused by measles are more prevalent in certain risk groups, including malnourished young children (below the age of 5) with vitamin A deficiency or compromised immune systems and pregnant women.
Overcrowded living conditions and lack of health infrastructure contribute to facilitating measles transmission and uncontrolled spread among these groups. Deaths and infections from measles in the developed world are becoming increasingly rare but remain a severe problem for children’s health in developing countries, where 10% of all infant deaths are due to measles.
The symptoms of smallpox and measles
Smallpox progresses slowly. The initial symptoms after the incubation period include:
After a few days, a characteristic rash appeared first on the face and then spread to other body parts. This rash included red spots filled initially with clear liquid and then thick pus. These pustules turn into a crust that dries and falls off, severely disfiguring and potentially blinding infected people. Ulcerating lesions also appear in the mucous membrane of the mouth and nose.
An infected individual remains contagious until all scabs fall off.
Measles tends to happen quickly and affects humans for a shorter period. After exposure to the virus, it begins its incubation in human cells for up to 2 weeks and only then do symptoms appear. Initial symptoms are very similar to smallpox, like high fever and a rash that spreads throughout the body. Other symptoms include:
In cases of severe measles illness, additional serious symptoms may develop. These complications may include:
severe diarrhoea and dehydration,
pneumonia,
blindness,
ear and sinus infections,
convulsions,
and encephalitis (brain-swelling infection).
These clinical features can eventually lead to death, especially in young children.
Conjunctivitis describes the inflammation of the inner eyelid layer and the white regions of the eyes. It is often referred to as Pink Eye.
The treatment of smallpox and measles
Smallpox, unfortunately, does not have a cure. The only known successful control measure against the variola-induced disease is vaccination.
The smallpox vaccine has been around for centuries and was the first vaccine to be successfully developed back in 1796!
Vaccination was crucial in eradicating the disease, but it is only effective if given up to four days after variola exposure. After identifying possible smallpox infections, the infected person’s household, along with the 30 surrounding households, would be vaccinated in a ring vaccination strategy. The ring vaccination and surveillance programs were the two successful strategies the WHO smallpox eradication program employed to stop outbreaks and end this infectious disease.
Fig. 2: Ring vaccination strategy.
The smallpox vaccine contains a virus closely related to variola called vaccinia. In rare cases, this smallpox vaccine can cause severe side effects and even be fatal. After eradicating smallpox, the vaccine stopped being administrated in routine immunisation programs and is only used in very specific cases.
These include, for example, researchers who may be currently working with the variola or a similar virus.
All smallpox-vaccinated individuals, who are primarily aged above 40, will have at least partial protection against variola infection that keeps them from experiencing more severe smallpox illness.
There are now new antiviral drugs developed for other diseases that might be successful in treating smallpox, making it less severe, but have never been tried because the disease has been eradicated. The same is not true for measles, which currently does not have any specific antiviral or other drug therapeutic treatments available.
Measles treatment typically only involves bed rest and medication to lower fever. The disease usually then clears away after a few weeks of symptom onset. In the rare case of severe measles, additional medication and treatments can be used to mitigate some of the most severe clinical features. These strategies usually include:
fluid intake for dehydration treatment,
antibiotics to treat pneumonia and ear infections,
vitamin A supplements, and other nutritional supportive care for young children with measles infections.
If these strategies are unsuccessful, measles infections and their associated complications can lead to death.
Much like smallpox, the only effective preventive measure to stop and eradicate measles is vaccination. Measles is a completely preventable disease. A two-dose inexpensive measles vaccine has been around for nearly 60 years and is the main tool required to eradicate this disease. Immunisation programs have steadily increased the percentage of people vaccinated for the past few decades.
The WHO data shows that in 2018, about 86% of all children worldwide had received at least one dose of the measles vaccine.
Despite the significant measles immunisation efforts, the disease infects and kills thousands of individuals because it is highly contagious. The WHO estimates a 93-95% immunisation coverage is necessary to stop community transmission and end this disease, which means all people must be vaccinated. It will most likely require many more years to achieve this goal in the developing world, as poor health infrastructure makes tracing infection contacts and vaccinating everyone harder.
Differences between smallpox, chickenpox and measles
Smallpox, chickenpox and measles are all three viral infectious diseases. As discussed before, smallpox is caused by the variola virus and measles by a virus of the paramyxovirus family.
Chickenpox is a viral infectious disease that affects humans and is caused by the varicella-zoster virus (VCZ).
Chickenpox, like measles, is still present and has not been eradicated. Despite being caused by different viruses, they cause similar rash and fever-like symptoms, often making them hard to distinguish from each other, leading to misdiagnosis.
Chickenpox and measles are both very contagious childhood infections and can be prevented through vaccination. Chickenpox is, however, far more common, infecting millions every year in developed countries. Both diseases result in the development of a rash, but they are different in appearance and growth. Chickenpox rash initially begins in the chest, face and back spreading from there, while measles’ rash typically starts in the forehead spreading downward. The rash itself is also different. Chickenpox rash is composed of papules (elevated red bumps), while measles has more flat-like spots.
Smallpox no longer occurs naturally, so many of its symptoms are now almost certainly always of another illness. Smallpox diagnosis is generally straightforward, and its symptoms are easy to recognise. Severe chickenpox however was sometimes misdiagnosed as smallpox before its eradication, but there are also slight differences between the rashes.
For example, chickenpox papules tend not to be as homogenous as smallpox.
Smallpox and Measles - Key Takeaways
- Smallpox is a deadly but eradicated viral infectious illness caused by the contagious variola virus.
- Measles is a viral infectious disease caused by a virus of the paramyxovirus family. Measles is one of the most contagious diseases and a leading cause of infant mortality in developing countries.
- Smallpox and measles are mainly caused by inhaling viral droplets released into the air by an infectious person’s breath. Both illnesses result in fever and rash-like symptoms.
- There are no antiviral treatments available for both diseases, but both are completely preventable through vaccination.
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