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The zero-g environment does cause some medical problems, but these are physiological, not pathological. In zero g your body experiences some cardiovascular deconditioning -- worse and faster if you're in great shape before you go, not so bad for lounge lizards -- and your bones start to lose their calcium. There are a few other undesireable effects of zero g, but they are all mechanical and changes in the body which could not be transmitted to another person.
When they put the Apollo 11 and 12 crews into quarantine, it wasn't fear of the zero-g environment but rather the fear they might import a virus from the moon. It's not inconceivable that a virus could survive in the near-vaccuum conditions of the moon; incredibly unlikely, but not inconceivable. A virus is a very simple arrangement of protein molecules; some viruses can survive outside a host for a very long time in very unfavorable circumstances without their proteins being broken down by radiation, vacuum, and temperature extremes.
A virus isn't really a life form. It doesn't reproduce itself; it reproduces by fooling a host's cells into making copies of it. Its survival lies in this simplicity. Most of the really damaging viruses, however, are more complex and cannot survive for long outside a host. Some even have to be transmitted to another host by riding along in the first host's cells. That's the trick the AIDS virus, HIV, uses; it can live in several body fluids, but blood and semen seem to be the primary vectors.
Now, a very simple virus, with simple, strong protein molecules, stands the best chance of survival in a hostile environment like deep space or on the surface of the moon. It's not much of a chance, but it's not impossible. One example in science fiction is The Andromeda Strain by Michael Crichton. But The Andromeda Strain was a science fiction monster, not reality.
I created a science fiction monster in my story "The Last Plague" (by Gregory Bennett, Analog, April 1995), but for exactly the opposite reason. Space, specifically zero g, was the solution to the problem in that novella. The characters needed to get into a zero-g laboratory to grow protein crystals from the virus which was wiping out warm-blooded life on Earth. The protein crystals allowed them to see the shape of the virus and determine the structure of its receptor sites; and that allowed them to select a serum which would work to combat it.
Some of the astronauts on the Apollo flights experienced an allergic reaction to lunar dust, but again, this isn't due to a pathogen and couldn't be transmitted to another person. Moon dust can excite hay-fever symptoms in some folks. That's because of the nature of the stuff. Earth dust has had all the rough edges weathered off, while moon dust still has its little microscopic jaggies intact. These pointy bits irritate the sinuses, causing the hay fever symptoms.
NASA has a health stabilization program for people assigned to an upcoming space flight, but it's not because they're worried the crew might catch something once they're in space. The concern is that they might carry something with them which would incubate into a full-blown disease during the space flight. That happened to the Apollo 7 crew; they carried a bad cold with them into space, and once was enough. On one Apollo flight, an astronaut was incubating a really nasty flu virus which hit him hard just after the flight started. Having a guy running a fever was bad enough, but the inconvenience of having diarrhea where the only potty you have is a plastic bag taped to your bottom can be a nightmare.
To avoid such things, a few weeks before a flight NASA identifies the Primary Contacts, folks who have to work close to the astronauts on that flight. Everyone else keeps their distance, and even the Primary Contacts have to get a little checkup every morning. It's a nuisance, but we're investing hundreds of millions of the taxpayers' dollars on every flight and we don't want to risk mission success on something which is easily avoided.
Bacteria, on the other hand, don't stand any reasonable chance at all. You'd have to find some really sensationalistic literature to worry about bacteria.
The Russians have done quite a number of biomedical experiments in space. They haven't been very rigorous in their documentation and controls so some of their research was either spoiled or didn't provide nearly as much information as it might have, but at least they've hatched some squid and even had a baby quail chick fluttering around aboard their space stations.
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