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Viruses -

There are millions upon millions of viruses on Earth. Luckily, there are only around 1,000 that are known to infect humans.

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Zoonotic viruses - The majority of these viruses are passed from animals to humans. They are called zoonotic viruses.

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COVID-19 -

New zoonotic diseases can crop up at any moment, just like with COVID-19.

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Infection -

A virus' goal is to infect the cells of its host and replicate.

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Infection -

The virus, also known as the pathogen, essentially hijacks the cell and forces it to infect other cells by continuously replicating.

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Infection -

If it spreads to enough cells, the host will eventually contract the virus.

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Virus habitat -

Many viruses only exist within the habitat where they naturally developed. This habitat could be an environment or a species of animal.

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Immunity -

For example, if a virus develops among a group of pigs, the pigs may never show any symptoms or be negatively impacted.

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Immunity -

Their immune system can control the infection without killing off the virus. The virus and the pig live in a sort of harmony!

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Lack of immunity -

However, if that virus jumps to a new species, they won't have the same immunity as the pig.

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Extremely rare -

Only a very small number of viruses manage to make this jump.

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Challenge -

This is because the virus has to pass through numerous barriers before it can infect the new species. It's not easy.

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Exposure -

There's a greater chance of this happening when the original carriers of the virus come in regular contact with another species.

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Exposure -

For example, a group of pigs on a farm will often interact with the farmers. This gives the virus many opportunities to attempt the jump.

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Transmission -

The virus makes its way to the farmer in a number of ways.

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Transmission - They can reach them through feces, slaughter, bites, or through an intermediary like a mosquito or tick.

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Those at risk -

This puts farmers and slaughterhouse and zoo employees, for example, at a higher risk of contracting viruses from animals.

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DNA - Their level of risk also depends on certain complex genetic factors. A long list of genes have been identified that can either make a person more susceptible or less susceptible to certain viruses.

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Defense -

Beyond genes, the virus has to make it past the body's immune system.

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Defense -

The human body has many levels of defense against disease. They are killed by stomach acid, mucus, T-cells, and antibodies.

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Mutation -

This is a challenge for the virus and it may need to mutate in order to complete its mission.

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Mutation -

When a virus mutates, it changes its genetic material. This can change the way it infects the human body.

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Mutation - A virus that attacks the digestive system of a pig may mutate and attack the lungs of a human, for example.

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Spread - Even if a virus manages to make it through all of the body's defenses, it may still only infect that individual.

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Spread -

Some viruses may jump to one human and leave it at that. Others may spread further.

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COVID-19 -

Some may spread so much that they lead to an epidemic or a pandemic, like with COVID-19.

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Survival -

Ironically, viruses do not want to kill their hosts because they also need the host in order to survive.

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The danger -

These zoonotic viruses are so dangerous because a human doesn't have the same antibodies as an animal. The virus is often much more dangerous to the human than it was to the animal.

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Unpredictable -

Another problem is that there are millions of animal viruses, and no way to predict which ones will make the jump to humans.

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Increasing risk -

The massive demands of our growing population are leading to more and more dangerous interactions with animals through mass farming and slaughter, deforestation, urbanization, and climate change.

See also: Civet cats bats and other animals who spread deadly diseases

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