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Of the 23 infectious diseases WHO lists as “epidemic and pandemic-prone”, 17 can spread from human to humans
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spinigera) living on monkeys. Here, people mostly depend on forests for a living (see ‘Forest fever’ on SARS and COVID-19 outbreaks have also been linked to exposure to the viruses in Chinese wet markets. Interconnectedness of the world has only made the spread massive and instantaneous.
It’s not just the interface with wildlife, livestock also plays a role. In the case of Spanish flu, it is widely held that the avian influenza virus jumped from a pig on a military farm in Kansas, USA, to the first known human case. Though there are other theories about where the “jump” took place, from Europe to China, what’s clear is that the virus mutated from animals and was taken across the world by the movement of soldiers during the World War I. Ultimately, the Spanish flu killed more people than the war.
So, it is a combination of factors— movement of people, living conditions, population density and, of course, eating habits—that makes the virus more deadly in its new host. Ebola, for instance, was not new to parts of Africa even though outbreaks were reported way back in 1976. What changed between then and the outbreaks of 2013-14 was the demography in the affected countries, says Sanath Muliya, a scientist with the Wildlife Institute of India, Dehradun. Between the 1960s and early 2010s, population density increased by 223 per cent in Guinea, 178 per cent in Sierra Leone and by 275 per cent in Liberia, particularly in the urban parts that experienced high rural-to-urban migration. All major outbreaks occurred in such urbanised set-ups with high human densities, says Muliya.
A similar devlopment in Indonesia in 1998-1999 led to the first outbreak of the Nipah virus infection, but in neighbouring Malaysia. The virus is naturally harboured by pteropid fruit bats. But in the months before the outbreak, large-scale deforestation was going on in Indonesia for pulpwood. Palm
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