Ebola

Okay, not the AMA guy, but I have some experience with infectious diseases (worked in lab at CDC). Wall o'text incoming.

Let me start of by explaining one thing: infection is a matter of probability, not of absolutes. Any given exposure has only got a certain chance of causing an infection, depending on the pathogen in question, time exposed, and the amount of pathogen you are exposed to.

As a general rule,the longer you are in contact with a pathogen, or the higher the dose of pathogen you get, the higher your odds of infection.

With that out of the way...

ONE: Route of infection is actually very important. Some viruses, like influenza, can easily get into the body through various routes. Other viruses, like norovirus, are only dangerous if you get them into your gastrointestinal tract. Still others, like HIV, require direct fluid contact for any transmission.

To put it another way, take this fake virus I've made up to illustrate the point.

Viral particles required to infect through blood contact: 1,500
Viral particles required to infect through mucosal membranes (eyes): 150,000,000
Viral particles required to infect through lungs: 15,000,000,000,000
Viral particles required to infect through gut: 15,000,000,000,000,000,000,000,000,000

An exposure of 1,500 particles is very likely but it can't infect unless it gets into direct contact with blood. However, aerosolized virus requires a MUCH higher dose to infect the lungs--it's possible, but requires a very long exposure or a very high dose. Gut infection is so unlikely as to be essentially impossible.

Similarly, while it may be theoretically possible for ebola to infect through the lungs, all of the evidence and the experiences of doctors/patients who've dealt with it suggest that it isn't very well suited to infecting people through the lungs or the gut (nobody that I've ever heard of has contracted the disease through infected water, for example).

Also, the dose of virus you receive from a microscopic droplet coughed up is minuscule compared to the dose you'd get from touching even a small droplet of blood coming directly out of the victim or through touching their feces. As we established earlier, dose matters.

TWO: Lab techs and doctors wear suits/have negative pressure rooms because ebola is exceedingly dangerous. The evidence suggests ebola isn't readily transmitted between humans by aerosol, but nobody is going to risk their lives just to prove it. Also, lab workers are more likely to receive a higher dose of airborne virus as they're working face-deep in an infected monkey's chest. Ditto for doctor's at a hospital working with ebola patients. They wear extra precaution because they're taking extra risk.

THREE: Pathogens behave very differently between different host animals. For example, bird flu is readily transmitted between birds but not to humans--only humans with prolonged contact with infected birds catch the disease. Evidence in monkeys and pigs is not directly applicable to humans, as I'm sure you're aware.

Second, I'd have to look at the studies you're referencing (links?), but I expect they're exposing the monkeys/pigs to very high doses of virus to ensure transmission.

FOUR: A couple of things here, but the biggest issue is that you're comparing a lab tech working with ebola for three or four hours at a time in a modern BSL-4 lab with doctors and nurses working 12-24 hour shifts in rural Africa. The African doctors have much less protection in general, are in contact with infected materials FAR longer at a time, and are going to be fatigued. When people are tired, they make mistakes--and when you're in contact with ebola virus for 12 hour stretches, that mistake can be fatal.

Ebola has ALWAYS been a danger to the doctors and nurses who care for patients because of limited resources and high stress. There's nothing new or unexpected about doctors/nurses getting sick--they're at the highest risk of anybody in the world simply because they're deliberately placing themselves into direct, prolonged contact with infected materials.

TL; DR:

1. route of infection matters
2. Ebola is one of the most dangerous pathogens known to medicine and has no treatment--nobody is going to work on it without taking all protective measures
3. Transmission in pigs and monkeys are suggestive, but not evidence that the virus can be highly infectious in humans lungs. This is especially true given the experiences derived from more than 40 years of surveillance.
4a. It's unreasonable to compare the infection rates of comfortable, well-rested lab techs in a BSL-4 lab with those of fatigued, stressed doctors and nurses working in a tent in rural Africa.
4b. There's nothing new or unexpected about doctors/nurses getting sick with ebola. They are probably the highest risk demographic as they are deliberately (and heroically) placing themselves in prolonged contact with the virus.