To understand the origins of the top five novel disease outbreaks in history according to death toll, including how they were transferred to humans.
Two of the top five novel disease outbreaks in history according to death toll are H1N1 in 1918-1919 and the HIV outbreak that began in the late 1970s. Other novel viruses include the Asian Flu (H2N2, 1.1 million deaths), Hong Kong Flu (H3N2, 1 million deaths), and SARS (774 deaths). Note that plagues were not included in this list as they are not considered "novel" and typically only transfer directly from animals to humans or animals to fleas or mosquitoes to humans (not humans to humans).
Outbreaks of H1N1 affected both Europe, Asia, and North America, with the first wave hitting the United States in March 1918. However, because outbreaks happened in Europe and Asia almost simultaneously, it is difficult to make a "definitive assignment of a geographic point of origin."
Genetic sequencing suggests that the H1N1 first infected humans at some point between 1900 and 1915, but that this infection likely had mammalian origins, probably swine, since "pigs can be infected with both avian and human virus strains."
So, on the way to the pandemic, birds transferred the virus to pigs and pigs then transferred it to humans. Unfortunately, scientists have no way of knowing "how long the virus may have been adapting in a mammalian host before emerging in pandemic form."
HIV is the virus that causes AIDS and has led to approximately 35 million deaths since it was first identified in 1983. It was originally called "HTLV-III/LAV (human T-cell lymphotropic virus-type III/lymphadenopathy-associated virus)," but it was later renamed HIV for human immunodeficiency virus".
The first known human case of HIV was in 1959 in a man living Kinshasa, Democratic Republic of the Congo. It is not known how he became infected, but "genetic analysis of [his] blood sample suggested that HIV-1 may have stemmed from a single virus in the late 1940s or early 1950s."
HIV first entered the United States in the mid-to-late 1970s and in 1982, the condition resulting from the virus began to be called "acquired immunodeficiency syndrome" or AIDS.
It is believed that the original transmission from chimpanzee to human occurred when "humans hunted these chimpanzees for meat and came into contact with their infected blood."
SIVs have been found in over 40 African primate species, but are "largely nonpathogenic in their natural hosts."
Since SIVs are only found in African primates, researchers believe that they likely "emerged in Africa sometime after the splits between lineages of African and Asian Old World monkeys, which are believed to have occurred around 6–10 million years ago."
HIV has four separate lineages, which are labeled M, N, O, and P, and group M is the line that is responsible for the global pandemic. Group O is mostly "restricted to Cameroon, Gabon, and neighboring countries." Group N is only present in 13 people in Cameroon, and group P has only appeared in two individuals: a Cameroon woman living in France and another person also from Cameroon.
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