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  • At a Maryland county fair in 2017,

  • the prize pigs were not looking their best.

  • Farmers reported feverish hogs with inflamed eyes and running snouts.

  • But while fair officials worried about the pigs,

  • the Maryland department of health was concerned about a group of sick fairgoers.

  • Some had pet the pigs, while others had merely been near their barns;

  • but soon, 40 of these attendees would be diagnosed with swine flu.

  • More often than not, sick animals don't infect humans.

  • But when they do, these cross-species infections,

  • or viral host jumps,

  • have the potential to produce deadly epidemics.

  • So how can pathogens from one species infect another,

  • and what makes host jumps so dangerous?

  • Viruses are a type of organic parasite infecting nearly all forms of life.

  • To survive and reproduce, they must move through three stages:

  • contact with a susceptible host, infection and replication,

  • and transmission to other individuals.

  • As an example, let's look at human influenza.

  • First, the flu virus encounters a new host

  • and makes its way into their respiratory tract.

  • This isn't so difficult, but to survive in this new body,

  • the virus must mount a successful infection

  • before it's caught and broken down by an immune response.

  • To accomplish this task,

  • viruses have evolved specific interactions with their host species.

  • Human flu viruses are covered in proteins

  • adapted to bind with matching receptors on human respiratory cells.

  • Once inside a cell, the virus employs additional adaptations

  • to hijack the host cell's reproductive machinery

  • and replicate its own genetic material.

  • Now the virus only needs to suppress or evade the host's immune system

  • long enough to replicate to sufficient levels and infect more cells.

  • At this point, the flu can be passed on to its next victim

  • via any transmission of infected bodily fluid.

  • However, this simple sneeze also brings the virus in contact with pets,

  • plants, or even your lunch.

  • Viruses are constantly encountering new species and attempting to infect them.

  • More often than not, this ends in failure.

  • In most cases, the genetic dissimilarity between the two hosts is too great.

  • For a virus adapted to infect humans,

  • a lettuce cell would be a foreign and inhospitable landscape.

  • But there are a staggering number of viruses circulating in the environment,

  • all with the potential to encounter new hosts.

  • And because viruses rapidly reproduce by the millions,

  • they can quickly develop random mutations.

  • Most mutations will have no effect, or even prove detrimental;

  • but a small proportion may enable the pathogen to better infect a new species.

  • The odds of winning this destructive genetic lottery increase over time,

  • or if the new species is closely related to the virus' usual host.

  • For a virus adapted to another mammal,

  • infecting a human might just take a few lucky mutations.

  • And a virus adapted to chimpanzees,

  • one of our closest genetic relatives, might barely require any changes at all.

  • It takes more than time and genetic similarity

  • for a host jump to be successful.

  • Some viruses come equipped to easily infect a new host's cells,

  • but are then unable to evade an immune response.

  • Others might have a difficult time transmitting to new hosts.

  • For example, they might make the host's blood contagious,

  • but not their saliva.

  • However, once a host jump reaches the transmission stage,

  • the virus becomes much more dangerous.

  • Now gestating within two hosts,

  • the pathogen has twice the odds of mutating into a more successful virus.

  • And each new host increases the potential for a full-blown epidemic.

  • Virologists are constantly looking for mutations

  • that might make viruses such as influenza more likely to jump.

  • However, predicting the next potential epidemic is a major challenge.

  • There's a huge diversity of viruses that we're only just beginning to uncover.

  • Researchers are tirelessly studying the biology of these pathogens.

  • And by monitoring populations to quickly identify new outbreaks,

  • they can develop vaccines and containment protocols to stop these deadly diseases.

At a Maryland county fair in 2017,

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