Every year, millions of people are infected with diseases you’ve probably never heard of.

There’s tons of media coverage about things like malaria, Zika, and HIV — all of which pose serious threats to public health. But there are many more diseases out there that are just as deadly -- yet not as infamous.

They’re called neglected tropical diseases, or NTDs, because historically they haven’t been paid as much attention as other diseases, and they mainly affect people in tropical areas. But these days, global healthcare initiatives are paying attention to them -- working to find ways to prevent and treat these diseases. The deadliest NTD — caused by the second-deadliest parasite in the world, after malaria’s — is schistosomiasis, also known as snail fever.

It’s caused by a parasitic worm, called schistosoma, that’s transmitted through water that’s been contaminated with the parasite’s eggs. The parasites use certain kinds of snails in the water as hosts, to grow and multiply. Eventually, the parasites’ larvae are released into the water, and that’s when anyone who comes in contact with the water is at risk, because the larvae can burrow through the skin.

Infected people often don’t have symptoms for the first month or two, though they can sometimes end up with a rash or other symptoms like fever, aches, and a cough. Symptoms are much more common later on, once the larvae have grown into adults and laid eggs. That’s when the infection is considered chronic.

But the symptoms of chronic schistosomiasis aren’t caused by the full-grown worms — the worms can actually incorporate proteins from the host to trick the immune system into ignoring them. It’s the eggs that cause an incredibly strong immune response, which can strike almost anywhere: the intestines, the lungs, the bloodstream, even the brain. So schistosomiasis can cause all kinds of symptoms, from diarrhea to wheezing to seizures.

More than 200 million people have schistosomiasis right now, and about 200,000 people die from it every year. The main way to treat it is to prevent it: if people are regularly given a drug called praziquantel, even if they don’t have the disease, they’re less likely to develop a severe infection later on. But the best way to protect people in the long term is to make sure communities have access to sanitation and safe drinking water, to stop the parasite from spreading.

Another neglected tropical disease is Chagas disease. It’s most prevalent in Central and South America, where about 8 million people have the infection, and about 10,000 die from it every year. Chagas is caused by a parasite called Trypanosoma cruzi, a single-celled protozoan.

The parasite spreads through the feces of infected insects, often known as kissing bugs. Usually, the parasite enters the body through mucous membranes — like in the eyes and mouth — or through broken skin, when someone scratches a kissing bug bite. The best time to treat the disease is just a few days after infection, but most people don’t show any symptoms for weeks.

And even in those who do, the symptoms are pretty mild and nonspecific — like fever, a rash, and diarrhea. And then something frightening happens: After this initial phase, Chagas infection usually goes dormant -- for years. After as much as a decade or two, about 30% of infected people then develop chronic, life-threatening symptoms — including swelling of the heart, or the intestinal tract.

So, prevention and early detection are very important here. But as with other NTDs, it often affects people who don’t have easy access to healthcare. Fortunately, efforts are underway to help those at risk, mainly with the help of insecticides.

If you kill the bugs, they can’t spread the disease. Then there’s African trypanosomiasis, otherwise known as African sleeping sickness, which affects parts of sub-Saharan Africa. Like Chagas, this disease is caused by parasites carried by insects — in this case, by the bite of the tsetse fly.

In the first phase of sleeping sickness, the parasite reaches the peripheral nervous system 3:42— the nerves that aren’t part of the brain or spinal cord. Here, symptoms include things like fever, joint pain, and itching, which can be caused by lots of different things. So — again like Chagas — sleeping sickness is hard to diagnose at first.

But symptoms become more severe when the parasite invades the central nervous system — the brain and spinal cord. Then, it can cause convulsions, confusion, and the symptom that gives the disease its name: uncontrollable sleepiness. If left untreated, sleeping sickness can be fatal.

But it is treatable with combined medications, even in the second stage. And initiatives to treat sleeping sickness have become more and more successful in recent decades. In 1995, there were an estimated 300,000 new cases of African sleeping sickness.

But since then, health organizations have been working on testing people for the disease and getting treatment to those who need it. They’ve also been using insecticides to kill tsetse flies. By 2014, there were fewer than 15,000 new cases of sleeping sickness.

There are lots of other NTDs out there as well. But they aren’t being neglected anymore, either, and that’s making a big difference. Thanks for watching this episode of SciShow, which was brought to you and sponsored by Bill Gates.

Visit GatesNotes.com to learn more about neglected tropical diseases and how people around the world are working to fight them -- like the Uniting to Combat NTDs Coalition, which recently set the world record for the most medication donated in 24 hours, with more than 200 million doses donated to prevent and treat NTDs.