It's not just humans that have to deal with epidemics. Disease outbreaks can kill thousands of animals very quickly. They hit especially hard if the animals are rare, threatened or fragmented species.
Over the last few decades many new animal diseases have emerged, and older diseases have spread to new areas. "Part of this is due to increased trade and travel, which brings pathogens to new regions," says A. Marm Kilpatrick of the University of California, Santa Cruz. Diseases are also being passed back and forth between humans, livestock and wild animals.
The biggest threat to wildlife is still loss of habitat, often triggered by the growth of farmland. Still, diseases can threaten wildlife populations with severe decline and extinction, says Richard Kock of the Royal Veterinary College in Hatfield, UK.
Here are ten diseases that are taking a heavy toll on wild animals. We'll start with a famous one.
We think of Ebola as a human disease, and with good reason. Last year's outbreak killed about 10,000 people. But it has also been wiping out populations of our closest relatives, the great apes.
In the early 1990s, the disease ravaged groups of chimpanzees living in Taï National Park in Côte d'Ivoire. The following decade, repeated outbreaks in the Republic of the Congo took a heavy toll on the gorilla population. Ebola killed about 5000 critically endangered western gorillas between 2002 and 2003 at the Lossi Sanctuary, and then wiped out hundreds of gorillas in the Odzala-Kokoua National Park in 2003-2004.
The ebola virus strikes both species hard. It kills about 95% of those it infects, by causing severe fever and bleeding.
Ebola's effects can be particularly devastating when combined with other threats like hunting and deforestation. Hunting has reduced the populations so much, the disease could cause extinction, says Julia PG Jones of Bangor University in the UK.
Vaccines against Ebola may be one way out. By 2014, researchers had tested a vaccine on a group of captive chimpanzees. It proved to be safe and effective.
A deadly fungus called "chytrid" has spelt doom for many frogs and salamanders. Over the last 30 years, it has sent more than 200 amphibian species into catastrophic declines, and in some cases driven them to extinction.
For instance, a wave of chytrid infections in Panama's El Copé in the early 2000s wiped out 30 species of amphibians. Five had never been seen before.
The fungus, called Batrachochytrium dendrobatidis, is found on every continent except Antarctica. It infects and damages the outer layer of the frogs' skin. Since frogs and salamanders use their skin to absorb nutrients and take in water, the infection eventually suffocates them.
But chytrid was not always the deadly killer it is today. For over 100 years, it coexisted harmlessly with amphibians in some places, such as Illinois and Korea.
Also, not all frogs infected with the fungus become sick or die. Some, like American bullfrogs and African clawed frogs, seem to be resistant. These species have been blamed for the spread of the disease, although Kilpatrick points out that the amphibian trade has also played a role.
3. West Nile encephalitis
In 1999, New York City became the epicentre of a new disease outbreak. People were admitted to hospitals with encephalitis: their brains were inflamed. Around the same time, several crows in the city, and other birds at the Bronx Zoo, turned up dead. In all cases, the culprit was the West Nile virus, which at the time was mostly found in Africa and Asia.
Transmitted by mosquitoes, the virus has since infected and killed millions of birds across the US, Mexico, and Canada. The virus has been detected in about 48 species of mosquitoes and 250 species of birds, and occasionally spills over to humans and horses.
In some areas, West Nile encephalitis has reduced the numbers of American crows by about 45%. It has also caused large declines in other bird species like American robins, eastern bluebirds, tufted titmouses and chickadees. But the West Nile virus does not likely threaten these species with extinction, says Kilpatrick.
Some endangered bird species are at more risk, however. So researchers have developed vaccines for critically endangered California condors, and for rare island scrub jays that are only found on California's Santa Cruz Island. Vaccines for other birds are also being tested.
4. White-nose syndrome
In 2006 a recreational caver took a photograph of a bat in a cave near Albany, New York. The bat had white fungus around its nose. This was the first evidence that a devastating disease had struck North America's bats. Called white-nose syndrome, the disease has rapidly spread across the US and Canada.
Nearly 6 million bats have been killed, and numbers of some species – like the northern long eared bat – have declined by 99% in the north-east. "White-nose syndrome is dramatically changing the bat populations of North America," says Kock.
The offending fungus is called Pseudogymnoascus destructans. It makes hibernating bats behave erratically. Instead of hibernating during the winter, the bats fly dangerously far out of their caves, even in daylight. As a result they quickly drain their fat reserves, and starve to death.
The fungus might have come from Europe, where it does not appear to harm the bats it colonises. Restricting human access to caves, and protecting their habitats, may help the bats.
Anthrax is best known for its use as a weapon of bioterrorism. But the disease is an ancient scourge of wildlife. It mostly affects herbivores, but anthrax can cause outbreaks in other mammals, including some carnivores, great apes and humans.
Anthrax can have different consequences based on the species and ecosystem. In places like Etosha National Park in Namibia, the disease is considered to be a natural part of the ecosystem, and has not been managed since the early 1980s, according to ecologist Wendy Turner of the University of Oslo in Norway.
Sometimes though, anthrax outbreaks can turn deadly. For instance, a 2004 outbreak in Zimbabwe's Malilangwe Wildlife Reserve killed over 90% of some of the wild herbivore populations. In 2010, a similar outbreak killed more than 80 hippos in Uganda.
Spores of the anthrax bacterium, Bacillus anthracis, can live in soil for several years and infect grazing livestock, and subsequently people. Regularly vaccinating livestock could restrict outbreaks, says Turner.
6. Devil facial tumour disease
A strange epidemic of contagious cancer has swept through Australia's Tasmanian devils. The cancer jumps from one devil to another when they bite each other. They do that a lot, while fighting over food or during mating.
The disease is often fatal. It results in large cancerous lumps on the devils' faces, which then spread over their entire bodies, killing them in a few months.
The disease is believed to have begun in a kind of nerve cell called a Schwann cell, in a single animal. Then the cancerous cells spread from one devil to another, helped by devils' tendency to bite each other.
Devils are all quite similar genetically, which means their immune systems are bad at fighting the cancer. It was first recorded as recently as 1996, but since then the disease has killed over 90% of the devils in some areas.
To preserve the species, scientists have established captive "insurance populations" of about 500 disease-free devils. These represent over 98% of the species' genetic diversity.
7. Canine distemper
A domestic dog virus is wiping out wild carnivores around the world. The virus, closely related to the human measles virus, attacks the animals' respiratory, nervous, and gastrointestinal systems.
In 1985, canine distemper slashed numbers of black-footed ferrets in Wyoming. Then in the early 1990s, the disease killed many African wild dogs and wiped out about 1000 lions from Africa's Serengeti. And in late 2000, the disease killed 49 of 52 captive African wild dogs in Tanzania within two months.
As domestic dog populations increase, the disease is spreading to newer areas, and jumping to a wider range of carnivores. For instance, the rare Amur tigers in eastern Russia have fallen prey to the virus.
Vaccinating domestic dogs against the virus can sometimes halt the spread of the disease. But it may not be enough as other animals can also spread it. Vaccinating the vulnerable wild carnivores might be the solution.
Australia's koalas are suffering from a sexually transmitted disease: chlamydia, which also affects humans. The disease can cause them to become infertile, or develop urinary and respiratory infections, to say nothing of blinding them and sometimes outright killing them.
Combined with drought, chlamydia has reduced koala populations in some areas of Australia from 60,000 in the mid-1990s to 10,000 in 2012. They have been worst hit in the states of Queensland and New South Wales.
To detect infections in time, some veterinarians have been scanning koalas using ultrasound machines, rather than taking swabs as usual. Scientists have also begun sequencing koala genes, including those that play an important role in the animal's immune system, to help figure out how the disease affects them.
The situation is being made worse by a second disease, a koala retrovirus similar to HIV. It suppresses their immune systems, making them more susceptible to chlamydia.
These two diseases, along with the destruction of their habitats and the threat from other animals, are pushing koalas towards extinction. But a vaccine, which has been successfully tested, offers some hope.
9. Sarcoptic mange
Sarcoptic mange causes severe itchingand an intense desire to scratch, leading to infections and sometimes death. It is caused by a parasitic mite called Sarcoptes scabiei.
The disease affects over 100 species, from wombats in Australia to red foxes and lynx in Europe and wolves in North America. A close relative of the mite infects humans, causing scabies.
A scabies mite burrows under an animal's skin, producing lesions that get infected. The infection spreads as the animal scratches the skin incessantly. Over time, the animal loses hair, becomes dehydrated, develops hypothermia, starves, and sometimes dies.
In many stable wildlife populations, mange does not appear to have a long-term effect on their numbers. But the disease can be catastrophic for populations that are already threatened, isolated or fragmented. For instance, sarcoptic mange is believed to have wiped out all the red foxes on the island of Bornholm in Denmark.
To establish mange-free animal pools, veterinarians sometimes treat them with drugs like ivermectin.
The same bacterium that causes plague in humans, including the infamous "Black Death" that struck Europe in the 1300s, also wreaks havoc in the animal kingdom. Confusingly it's called "sylvatic plague" but that just means it's a disease of wildlife. It's all the same microorganism: Yersinia pestis.
Sylvatic plague first appeared in North America around 1900. Ships coming from plague-ridden areas of Europe and Asia probably carried over infected rats and fleas, which passed it on to the local wildlife. These animals had never been exposed before.
In some areas plague wiped out entire colonies of prairie dogs. With mortality rates over 90%, sylvatic plague has been devastating for these animals.
This massive prairie dog die-off has in turn caused black-footed ferret populations to plummet. One of North America's rarest animals, these ferrets feed mainly on prairie dogs, and use their burrows to raise their young. So when prairie dogs die, ferret populations decline too. Moreover, the disease is almost always fatal to the ferrets.
The endangered ferrets have been bred in captivity and reintroduced, and their populations are slowly recovering. Vaccinating the ferrets may also help prevent outbreaks. Similarly, dusting prairie dog burrows with pesticides and vaccinating them using baits might help protect prairie dog populations.