" if microbial life is indeed found on Mars, or even our own moon, that they don't try and bring it right back to Earth...I don't really fancy being exterminated by an unrelenting bacterial infection... "
The reason that certain types of bacteria 'infect' humans (and other animals) is that they have co-developed over millions of years. Bacteria (and viruses) that cause human illnesses are highly evolved to take advantage of specific characteristics of human physiology. It is extremely unlikely that any alien bacteria would cause widespread and fatal epidemics except by pure chance. (not that we should be betting the survival of the species on chance, however small)
Sorry HG Wells, your plot twist wouldn't work in real life :)