Our relationship to the animal kingdom is often wonderful and even life-saving — it provides us with everything from food to companionship to insulin — but there’s a dark side too. The vast array of bacteria, viruses, and other microscopic pathogens that threaten our health can be found everywhere, sure, but many of our worst germs are caught from the cute creatures and creepy crawlies that occupy our world alongside us.

Here now is a brief look at five of the deadliest diseases we get from animals.

Cow
Medical Daily takes a look at some of the deadliest diseases to come from animals. Pixabay, Public Domain

Ebola

Ebola’s long been a viral disease whose bark was much worse than its bite since it was first discovered in 1976.

That all changed with the recent widespread epidemic of Ebola that engulfed West Africa and even briefly reached the shores of America. Starting in 2013, the outbreak is estimated to have infected nearly 30,000 people, and killed more than 11,000. And though the worst of it ended by 2015, it was only earlier this summer that Liberia and Guinea — two of the hardest hit countries — were declared Ebola-free. The outbreak may have been fueled by a mutation that allowed the virus to spread faster in people than ever before.

Spread by infected bodily fluids, Ebola’s fatality rate can differ from 25 percent to 90 percent, depending on the strain of virus and level of supportive care, according to the World Health Organization. Although researchers are hard at work trying to develop an effective treatment, there’s no readily available one yet. And while we’re sure that Ebola’s origins are found among Africa’s wildlife, we still don’t know for certain where its natural home is. Currently, the prime suspects are fruit bats, but outbreaks have likely been started through contact with infected or dead chimpanzees, gorillas, and other primates as well.

As previously reported by Medical Daily, the virus is now also suspected to be transmissible through sex, possibly even after someone is no longer actively sick.

Rabies

Usually transmitted by the bite of an infected mammal like a bat or dog through its saliva, the viral rabies is often seen as a bogeyman of the past. But while cases are rare in the developed world, they do still occur. Globally, the virus kills around 56,000 annually, all while racking up an economic toll of $8.9 billion, Medical Daily previously reported.

Technically, rabies isn’t as universally fatal as it’s commonly portrayed. Victims can still stave off death by obtaining the cheap and easily available vaccine soon after infection. Wait around a few weeks for its telltale symptoms, which include hallucinations and excessive salivation, to show up, however, and your chances of survival are practically nil. At that point, the fatality rate is upwards of 99 percent as the virus devastates the nervous system and eventually shuts down your organs.

Malaria

Malaria's fearless reputation has dimmed somewhat in light of other similarly transmitted diseases like Zika making the front page news more recently. But make no mistake, malaria has been and is by far the deadliest disease to be spread by mosquitos.

Just in 2015 alone, the family of parasitic protozoa responsible for malaria caused a whopping 214 million cases and killed over a half million people worldwide, according to the Centers For Disease Control And Prevention. There are 4 major species of Plasmodium spread via mosquito that cause malaria, with P. falciparum being the deadliest. Another species, P. knowlesi, isn’t spread by mosquitos but can be transmitted from certain kinds of macaque monkeys throughout Southeast Asia.

Though many treatments exist, there are strains resistant to them, and the disease can also return with a vengeance thanks to dormant cysts that hide inside the liver.

Mad Cow Disease

Neither living nor a virus, prions are the misshapen form of proteins readily found in our noggins already. When prions are next to these normal proteins, they convert them into more prions, like a microscopic Agent Smith from The Matrix. As prions accumulate, they warp the normal structure of the brain, causing telltale holes that make it resemble a sponge. These prion diseases, or spongiform encephalopathies as they’re formally called, can be transmitted by contact with infected bodily fluid or brain matter, but may also sprout up randomly due to genetic mutations, and are 100 percent fatal, according to the CDC.

Mad cow disease, or bovine spongiform encephalopathy, is an even stranger prion disease than others. As the name indicates, it’s found in cows. But a number of people who ate contaminated beef in the 1980s and 1990s developed their own version of mad cow, called variant Creutzfeldt–Jakob disease. The cows themselves had likely contracted it from eating the meat of sheep infected with their own version of a prion disease called scrapie. More than 200 people worldwide contracted mad cow during this time period, though more cases are still appearing since prion diseases can take decades to show up, Medical Daily previously reported.

Thankfully, newer cases of mad cow have since become rare, thanks to a widespread ban on feeding cows meat and better slaughterhouse practices. In people, prion diseases remain rare, though again, there have been cases of variant Creutzfeldt–Jakob disease sparked by mutation rather than eating contaminated meat. There are also types of mad cow that also occur spontaneously, though it isn’t known if people can become sick from eating beef contaminated with these prions.

Influenza

There are actually two major types of the influenza virus, A and B.

The B type is what most people think of when they think of the flu. Influenza B viruses only call our bodies home and spread easily from person to person. Influenza A viruses, however, are found in other mammals naturally, such as birds, pigs and even bats, as well as people and can be more much dangerous.

That’s because, under certain circumstances, different strains of A viruses can mingle with one another and swap genetic material. That can create a perfect storm of disease, such as a new flu virus that is both deadly and quick-spreading in humans. The most dramatic example of this was seen during the 1918-1919 pandemic flu outbreak, which killed 50 to 100 million people worldwide, according to the CDC. The latest in 2009, an avian flu, killed 12,000 Americans and infected another 60 million.

Thankfully, these pandemic flus are far and few between, though scientists remain ever vigilant against their appearance. Current seasonal flu vaccines vaccinate against both A and B viruses.