A pandemic is the outbreak of disease on a global scale.There are many examples in history, the most recent being the COVID-19 pandemic, declared as such by the World Health Organization on 12 March 2020.
Pandemics are generally classified as epidemics first, i.e. the rapid spread of a disease in one or more specific areas. The Zika virus epidemic that started in Brazil in 2014 and spread across the Caribbean and Latin America was an epidemic, as was the Ebola epidemic in West Africa in 2014-2016.
According to the U.S Department of Health and Human Services, the United States has been experiencing an opiate epidemic since 2017 because of the abuse and high number of deaths caused by the drug.
COVID-19 started as an epidemic in China, before making its way around the world in a few months and becoming a pandemic. But epidemics don't always become pandemic, and the transition is not always quick or clear. For example, HIV was considered an epidemic in West Africa for decades before becoming a pandemic at the end of the 20th century. Today, thanks to advances in modern medicine, HIV is considered endemic, which means the rate of the disease is stable and predictable in certain populations, according to the American Medical Association.
Modern influenza pandemics
Most viral pandemics have been caused by influenza viruses. Influenza viruses can change from season to season, and if health professionals are good enough to predict how the virus will evolve, sometimes a new virus will appear and not behave as expected. This is when a pandemic is most likely to occur because most people are not immune to the new virus.
The deadliest pandemic in history was the Spanish flu of 1918. The virus infected one-third of the world's population and caused between 20 and 50 million deaths, an estimated mortality rate of 1-3%. The virus did not originate in Spain, but the country was thought it started in France and called it "French flu".
The 1957-58 Asian influenza pandemic was triggered by a new strain of influenza. A virus (H2N2) that appeared in East Asia, according to the Centers for Disease Control and Prevention. The virus killed an estimated 1.1 million people worldwide, which corresponds to an estimated mortality rate of 0.019 percent, according to a study in The Journal of Infectious Diseases.
The 1968 Hong Kong influenza pandemic was caused by a new strain of the H3N2 virus that emerged in Southeast Asia. Once again, the pandemic earned its name because of the origin of the first information about the outbreak, not because of the origin of the virus. According to CDC estimates, the Hong Kong flu has killed one million people worldwide, about 0.03% of the world's population.
The H1N1 swine flu pandemic of 2009-2010 was caused by a new strain of the same virus that caused the Spanish flu- the H1N1 virus. It is estimated that swine flu infected between 700 million and 1.4 billion people, which is much more in absolute terms than the Spanish flu. But the mortality rate was much lower, estimated at between 0.01% and 0.08%, according to an analysis published in The Lancet.
Seasonal influenza is a year-round disease worldwide and although the vaccine is effective, deaths from influenza-related is generally responsible for 290.000 to 650.000 deaths per year.
Other notable pandemics in history
One of the most famous pandemics in human history was the Black Death, a worldwide epidemic of bubonic plague that occurred between 1346 and 1353. The disease is caused by the bacterium Yersinia pestis and caused the death of 30-60% of the European population in the mid-14th century, although experts believe that the disease appeared in Central Asia decades earlier.
The first cholera pandemic occurred in 1817 and originated in Russia, where one million people died, according to History.com. The bacterium was transmitted to British soldiers, who transported it to India and eventually to the rest of the world.
The Russian flu of 1889 is considered to be the first major flu pandemic. It probably started in Siberia and Kazakhstan before spreading west in Europe and crossing the Atlantic Ocean to reach North America and later Africa. By the end of 1890, an estimated 360,000 people died from Russian flu, according to History.com.
LHIV, the virus that caused AIDS, probably developed from chimpanzee virus that was transferred to humans in West Africa in the 1920s. The virus made its way around the world and HIV/AIDS was a pandemic by the end of the 20th century. An estimated 35 million people have died from the disease since its discovery, but drugs developed in the 1990s now allow people with the disease to stay alive with regular treatment. Even more encouragingly, two people were cured of HIV in early 2020.