Rabies. It's exceptionally common, but people just don't run into the animals that carry it often. Skunks especially, and bats.
Let me paint you a picture.
You go camping, and at midday you decide to take a nap in a nice little hammock. While sleeping, a tiny brown bat, in the "rage" stages of infection is fidgeting in broad daylight, uncomfortable, and thirsty (due to the hydrophobia) and you snort, startling him. He goes into attack mode.
Except you're asleep, and he's a little brown bat, so weighs around 6 grams. You don't even feel him land on your bare knee, and he starts to bite. His teeth are tiny. Hardly enough to even break the skin, but he does manage to give you the equivalent of a tiny scrape that goes completely unnoticed.
Rabies does not travel in your blood. In fact, a blood test won't even tell you if you've got it. (Antibody tests may be done, but are useless if you've ever been vaccinated.)
You wake up, none the wiser. If you notice anything at the bite site at all, you assume you just lightly scraped it on something.
The bomb has been lit, and your nervous system is the wick. The rabies will multiply along your nervous system, doing virtually no damage, and completely undetectable. You literally have NO symptoms.
It may be four days, it may be a year, but the camping trip is most likely long forgotten. Then one day your back starts to ache... Or maybe you get a slight headache?
At this point, you're already dead. There is no cure.
(The sole caveat to this is the Milwaukee Protocol, which leaves most patients dead anyway, and the survivors mentally disabled, and is seldom done).
There's no treatment. It has a 100% kill rate.
Absorb that. Not a single other virus on the planet has a 100% kill rate. Only rabies. And once you're symptomatic, it's over. You're dead.
So what does that look like?
Your headache turns into a fever, and a general feeling of being unwell. You're fidgety. Uncomfortable. And scared. As the virus that has taken its time getting into your brain finds a vast network of nerve endings, it begins to rapidly reproduce, starting at the base of your brain... Where your "pons" is located. This is the part of the brain that controls communication between the rest of the brain and body, as well as sleep cycles.
Next you become anxious. You still think you have only a mild fever, but suddenly you find yourself becoming scared, even horrified, and it doesn't occur to you that you don't know why. This is because the rabies is chewing up your amygdala.
As your cerebellum becomes hot with the virus, you begin to lose muscle coordination, and balance. You think maybe it's a good idea to go to the doctor now, but assuming a doctor is smart enough to even run the tests necessary in the few days you have left on the planet, odds are they'll only be able to tell your loved ones what you died of later.
You're twitchy, shaking, and scared. You have the normal fear of not knowing what's going on, but with the virus really fucking the amygdala this is amplified a hundred fold. It's around this time the hydrophobia starts.
You're horribly thirsty, you just want water. But you can't drink. Every time you do, your throat clamps shut and you vomit. This has become a legitimate, active fear of water. You're thirsty, but looking at a glass of water begins to make you gag, and shy back in fear. The contradiction is hard for your hot brain to see at this point. By now, the doctors will have to put you on IVs to keep you hydrated, but even that's futile. You were dead the second you had a headache.
You begin hearing things, or not hearing at all as your thalamus goes. You taste sounds, you see smells, everything starts feeling like the most horrifying acid trip anyone has ever been on. With your hippocampus long under attack, you're having trouble remembering things, especially family.
You're alone, hallucinating, thirsty, confused, and absolutely, undeniably terrified. Everything scares the literal shit out of you at this point. These strange people in lab coats. These strange people standing around your bed crying, who keep trying to get you "drink something" and crying. And it's only been about a week since that little headache that you've completely forgotten. Time means nothing to you anymore. Funny enough, you now know how the bat felt when he bit you.
Eventually, you slip into the "dumb rabies" phase. Your brain has started the process of shutting down. Too much of it has been turned to liquid virus. Your face droops. You drool. You're all but unaware of what's around you. A sudden noise or light might startle you, but for the most part, it's all you can do to just stare at the ground. You haven't really slept for about 72 hours.
Then you die. Always, you die.
And there's not one... fucking... thing... anyone can do for you.
Then there's the question of what to do with your corpse. I mean, sure, burying it is the right thing to do. But the fucking virus can survive in a corpse for years. You could kill every rabid animal on the planet today, and if two years from now, some moist, preserved, rotten hunk of used-to-be brain gets eaten by an animal, it starts all over.
So yeah, rabies scares the shit out of me. And it's fucking EVERYWHERE. (Source: Spent a lot of time working with rabies. Would still get my vaccinations if I could afford them.)