For humans, rabies is pretty far up the list. 99.99999% mortality, and a slow, agonizing, mind-destroying death. We do have a vaccine, which if given early enough (hours or days), has a very high cure rate.
Ebola, and other hemorrhagic fevers are also pretty bad. The virus factory is unrestrained, so little bricks of virus pile up and rupture cells. Bleeding from the eyes, and other body parts. People don’t exactly liquefy, but it’s almost as bad. Mortality ranges from 10–90% depending on the strain.
Cholera is pretty bad, in that you have diarrhea so badly that you die of dehydration. Antibiotics and IV fluids greatly reduce the mortality though.
AIDS is particularly nefarious. It is transmitted by any bodily fluid, though mostly blood. Sex usually causes micro-tears, which allow it through the epithelial tissue, and that’s why it’s primarily categorized as an STD; however, prior to good screening, it was also transmitted by blood transfusions. And then it sits, waiting. It stays active enough to spread, sometimes for a decade or more. Then, it kicks in, and shuts down key parts of your immune system. Things that we all carry then become horrible diseases that your body simply cannot recover from. Antibiotics and antivirals do not help, because they do not cure you. They help your body have time to cure itself. Luckily, treatment has come a long way, and includes reduction, suppression, and in limited, high risk situations, possibly a cure. There is no wide-spread cure though, just a few people found to have natural immunity that can be passed on through transfusions.
Cordyceps is a horrible disease, in that it is a fungus that infects the host, infects the brain, and causes the host to climb up high, and root in, so that the fungus can sprout through the head, then rain spores down on the others. Luckily, none of the strains of it seem to affect humans, but various other animals, from ants to spiders, can be infected.
Some people will say humans are the worst disease to infect the Earth. This is arguable, but we do have a marked impact, and have trouble cleaning up our own waste.
On the opposite end of the spectrum, the BEST diseases, in my opinion, are the purple sulfur oceanic bacteria that tried to infect eukaryotes, and got trapped. They provided a tremendous amount of cellular energy, and lost the ability to move or even leave the cells. I’m talking about mitochondria, and chloroplasts. These little powerhouses make complex life of our variety possible. Without them, energy production is massively less efficient, and only the most simple of cellular colonies can form.
