Scientists discover the origin of smallpox

Smallpox was once one of humanity’s most devastating diseases, but its origins are shrouded in mystery. (CREDIT: Creative Commons)

Smallpox was once one of humanity’s most devastating diseases, but its origins are shrouded in mystery. For years, scientific estimates of when the smallpox virus first appeared have diverged from historical records. Now a new study shows the virus appeared 2,000 years earlier than scientists had previously shown, checking historical sources and confirming for the first time that the disease has plagued human societies since ancient times.

The article was published in the journal Microbial Genomics, published by the Society for Microbiology.

Smallpox, caused by the variola virus, is perhaps best known as the only human infectious disease to have been eradicated worldwide. But the disease was the leading cause of death until relatively recently, claiming the lives of at least 300 million people in the 20th century. This is roughly equivalent to the population of the United States.

Until relatively recently, the earliest genetic evidence for smallpox was only from the 1600s. Then, in 2020, a study that sampled the skeletons and teeth of Viking Age skeletons identified multiple strains of smallpox and confirmed the existence of the virus for at least another 1,000 years.

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However, some historians believe that smallpox existed long before the Vikings. Suspicious scarring on ancient Egyptian mummies (including Pharaoh Ramses V, who died in 1157 BC) leads some to believe that smallpox has a history of at least 3,000 years. Until now, the missing scientific evidence to support this theory has remained hidden.

By comparing the genomes of modern and historical variola virus strains, researchers at the Eugenio Medea Scientific Institute and the University of Milan in Italy traced the evolution of the virus into the past. They found that all the different strains of smallpox descended from a single common ancestor and that a small fraction of the genetic components found in Viking Age genomes persisted into the 18th century.

They also determined the time the virus emerged. In their assessment, the researchers took into account what is called the “time-dependent speed phenomenon.” This means that the rate of evolution depends on the time span over which it is measured, so viruses seem to change faster over a short period of time and slower over a longer period of time. This phenomenon has been well documented for DNA viruses such as smallpox.

Using a mathematical equation, scientists can explain the rate versus time phenomenon to give more precise dates for evolutionary events, such as the emergence of a new virus. This gave the team a new estimate for the first appearance of smallpox: more than 3,800 years ago. As historians have long suspected.

The researchers hope that these results will resolve a long-standing dispute and provide new insights into the history of one of humanity’s deadliest diseases.

Smallpox virus. (Credit: iStock/Peddalanka Ramesh Babu)

“The smallpox virus may be much older than we thought,” said Dr. Diego Forni, first author of the study. “This is important because it confirms the historical hypothesis that smallpox existed in ancient societies. It is also important to consider that there are some aspects to the evolution of viruses that should be taken into account when doing this kind of work.”

Smallpox symptoms

According to the Mayo Clinic, the first symptoms of smallpox usually appear 12 to 14 days after exposure to the smallpox virus. However, the virus can stay in your body for 7 to 19 days before you feel sick. This time is called the incubation period.

After an incubation period, sudden flu-like symptoms occur. These include:

  • Heat

  • Muscle pain

  • Headache

  • severe fatigue

  • Severe back pain

  • Vomiting, sometimes

After a few days, flat red spots appear on the body. They can start in the mouth and on the tongue and then spread to the skin. The face, arms, and legs are often affected first, followed by the trunk, hands, and feet.

Within a day or two, many of the spots turn into small blisters filled with a clear liquid. Later, the blisters fill with pus. These ulcers are called pustules. Scabs form after 8-9 days and eventually fall off, leaving deep, pitted scars.

Smallpox can be passed from person to person when the rash appears and until the crusts fall off.

Causes of smallpox

Smallpox is caused by the variola virus. The virus can spread:

  • Directly from person to person. You can catch the smallpox virus by being around someone who has it. An infected person can spread the virus by coughing, sneezing, or talking. Contact with skin sores can also lead to smallpox.

  • Indirectly from an infected person. In rare cases, smallpox can spread through the air inside buildings, infecting people in other rooms or on other floors.

  • through contaminated items. Smallpox can also be transmitted through contact with contaminated clothing and linens. But it is less likely to contract smallpox this way.

  • As a terrorist weapon, potentially. The use of smallpox as a weapon is unlikely. But since the release of the virus could lead to the rapid spread of the disease, governments are preparing for the possibility.

Complications from smallpox

Most people who get smallpox survive. However, some rare types of smallpox are almost always fatal. These more severe forms are most common in pregnant women and children.

People recovering from smallpox usually have severe scars, especially on the face, arms, and legs. Smallpox sometimes causes loss of vision (blindness).

To learn more about science and technology, visit our New Discoveries section at The bright side of the news.

Note. Materials provided by the Society for Microbiology above. Content can be edited for style and length.

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