(CNN) It is a slow and cumbersome descent hindered by head-to-toe protective suits. A wooden ladder dips down the side, as two researchers in white suits and respirators make their way into Grootboom cave in South Africa.
The suits make the journey more cumbersome, but they are the only things standing between safety and exposure.
Inside the cave, thousands of bats either cling tightly to the walls or fly around in bunches. Any number of them could contain deadly pathogens.
Virus hunters search for the next deadly disease outbreak
Two researchers carefully descend into Grootboom cave, located just miles away from the densely populated city of Johannesburg in South Africa.
The researchers from the University of Pretoria and the Centers for Disease Control and Prevention (CDC) are tracking animals all around the world to create an early-warning system for diseases that could affect humans. Here, they hunt for bats inside Grootboom cave.
The cave is inhabited by thousands of bats, any of which could be carrying deadly diseases such as Ebola, Marburg or rabies.
There are 1,240 species of bats worldwide, which may explain why many viruses can be found inside them.
Without the suits, the researchers are vulnerable to any potential zoonotic diseases the bats could be harboring, which could transmit from animals into humans.
In this particular part of the cave are long-fingered bats. Wanda Markotter, a researcher with the University of Pretoria, catches ones inside a net and slowly stretches out its wing.
"This specific species has been implicated in some of the rabies-related viruses," Markotter says. "They are in high-population densities, and people are going to come in close contact with them."
10 diseases you thought were gone
In the 19th century, scarlet fever was a common killer in Europe. In 2016, nearly 20,000 cases were reported in the United Kingdom -- the biggest increase in 50 years.
Scarlet fever is just one disease that many have forgotten but that is by no means gone, despite our best efforts to eliminate it.
Though it's often thought of as a medieval disease, the World Health Organization reports 1,000 to 3,000 global cases of plague every year. The US averages seven new human cases a year,
mostly in the Southwest. Pictured, a patient with gangrene and necrosis, caused by plague.
This 19-year-old girl in Neijiang, China, is being carried in a basket because she has rickets. Rickets is caused by a lack of vitamin D, which we get from sunlight. Experts believe rickets is making a comeback in developed countries because of the use of sunscreen and less time spent outdoors.
Gout was once known as the "disease of kings" because of its links to excessive food and alcohol consumption. These days, unhealthy lifestyles are behind an increase in gout in developed countries.
The deadly disease diphtheria affects the nose and throat. Vaccinations mean it is now rare in developed countries, but in 2011, nearly 5,000 cases were reported to the WHO, with many more probably unreported.
There were 216,000 global cases of leprosy, an ancient and disfiguring disease, in 2013.
TB, or tuberculosis, is one of the top 10 killers in the world, according to the WHO. In 2016, 10.4 million people around the globe contracted tuberculosis, and more than 1.7 million died. Ninety-five percent of those
deaths were in developing countries.
Vaccination programs mean polio is on the verge of extinction -- but there are still cases in Afghanistan and Pakistan.
That's the other important part about this location. This is not some remote cave; this is only miles away from the densely populated of city of Johannesburg.
"There are 1,240 species [of bats] worldwide," Markotter says. "So they represent a big group of mammals and that's why we probably find a lot of viruses in them."
Researchers are tracking animals here in South Africa, and all around the world, as a form of early-warning system for diseases that could come to humans. It is widely believed, for instance, that the first cases of Ebola began with a bat.
In 2014, West Africa saw an unprecedented outbreak of the Ebola virus, with cases concentrated in Sierra Leone, Liberia, and Guinea. The disease was first discovered in 1976 in what is now the Democratic Republic of Congo and there have been multiple outbreaks since that were well-controlled, but today things are more challenging. In a time where international travel has connected the world more than ever before, it takes a coordinated, global effort to prevent a widespread outbreak -- as the 2014 outbreak showed us.
Much of that coordination happens at the headquarters of the Centers for Disease Control and Prevention in Atlanta, Georgia. The CDC oversees 10 global disease detection centers around the world, including in South Africa, where constant monitoring is taking place.
"We were tracking almost 300 infectious disease outbreaks of concern in 145 countries," says Dr. Jordan Tappero, director of the Global Health Protection Center at CDC. This was during a 2-year period.
"Only about 30% of countries even self-report [and] are able to prevent, detect, and respond to infectious disease outbreaks," Tappero says. "We are working around the world to try and improve capacity so that we have partners everywhere to respond quickly."
Diseases you can catch from animals
Roundworm is one of the most common diseases that we get from dogs. Every year there are about 10,000 cases of roundworm spreading through the body and causing fever and fatigue. Dogs and puppies also can transmit campylobacter infections. Symptoms include diarrhea, cramping, abdominal pain and fever.
Some of your feline's favorite activities, licking and scratching, can give you the bacterium that causes cat scratch fever, which leads to swollen lymph nodes. Cats also can transmit campylobacter infections, causing symptoms of diarrhea, cramping, abdominal pain, and fever.
About 70,000 people get salmonella infections, typically including fever and diarrhea, from reptiles every year in the US. The bacteria can live on reptiles, like turtles, without making them sick.
Parrot fever can infect parrots, parakeets and macaws, but rarely causes symptoms for them. It can, however, cause fever, chills, headache and pneumonia in people.
Some armadillos are naturally infected with leprosy, also known as Hansen's disease, according to the Centers for Disease Control and Prevention. It is possible, though unlikely, for humans to catch the disease from armadillos.
Since January of last year, the US Centers for Disease Control and Prevention (CDC) reported at least 20 people in the U.S. came down with salmonella infections linked to crested geckos they brought home from pet stores.
Hamsters and other "pocket pets" can carry lymphocytic choriomeningitis virus (LCMV). The virus typically causes flu-like symptoms, including fever and muscle aches.
Petting zoos and county fairs have been associated with outbreaks of E. coli and flu.
That monitoring starts with teams on the ground in caves like Grootboom.
The team now crawls through a narrow gap to reach a difference chamber, where different species of bats with other potential pathogens live. Each bat has the potential to carry rabies, Marburg, even Ebola. These are some of the planet's deadliest, but least understood, viruses.
The risk is justified by the importance of the work as it is in the interest of public health, says Markotter.
"If you don't know what's in the animals, you won't be able to identify a human outbreak quickly," she says. "Most of the time we only respond when there are lots of human deaths and we don't know where the virus came from, or where the pathogen came from."
Outside the mouth of the cave, a makeshift lab comes to life under a white pop-up tent. Bats collected from various parts of the cave are tested, samples are collected from their mouths, fur, and wings and everything is meticulously recorded. This is all part of the constant monitoring and early warning systems.
If the team knows what's circulating in the bats, they can flag it and alert the population to take precautions, or watch for certain symptoms. Their efforts will matter to keep this closely-connected world healthy.
CNN's David McKenzie and Brent Swails contributed to this report.