The more something is like ourselves, the harder it is to attack it with chemicals. Our cells are very similar in nature to a lot of protozoan parasites, so the drugs that may treat them are pretty toxic to ourselves. Bacteria and fungi are much less like our cells, and they have a lot of unique features such as peptidoglycan or ergosterol in the cell wall, therefore they can be attacked with chemicals that are less toxic. Animals, such as insects and nematodes, that have diverged significantly away from our common origin, have evolved new features that we do not share, and we can attack those. Some organisms of interest to (veterinary) medicine, such as Leishmania and Pythium, are very difficult to treat.

This is also why Cancer is very difficult to treat. Because it is your cells.

Chemo is poison that effects cancer cells slightly more severely then healthy cells.

After taking a lot of medical courses just for fun, and a lot of neuroscience, I took a course "Introduction to Clinical Neurology" offered on Coursera that had some stringent exam conditions so that it would even count towards having taken an official course for practicing doctors as part of continuing education. I also took one on the same platform actually taught by African doctors about various parasites common in Africa, leishmaniasis was in it too. (Note: Can't link, after the reorg. of Coursera those free courses are not even available in archived form.)

So anyway, especially the neurology course was kind of depressing. Basically, apart from watching and treating a few symptoms, and maybe delaying symptoms (the main example being Parkinson's and brain implants), there is nothing that can be done.

Looking at the state of medicine I'm reminded of the movie "City of Ember" (http://www.imdb.com/title/tt0970411/), where survivors of the nuclear apocalypse were placed in an underground city, and the movie starts a few hundred years later when their (1960s level) technology and especially their power plant starts falling apart after many, many repairs. The engineers there, with no access to make anything new, only the supply they were given at the beginning, are like doctors, and the machinery like human bodies. You start with what you are given and it all goes downhill from there, with more or less horrible kludges and hacks around arising problems along the way.

There are sadly still a lot of things we treat roughly in the same way, from most unoperable cancers to even strong acne (isotretinoin may not be "poison" in the commonly understood way, but the underlying concept of how and why we take it is still pretty much the same).

When you see scifi movies and shows where any ailment can be cured with 5 minutes in a machine, it's still a far away dream from where we are now. In a way, it's also full of hope.