So if aliens really did land...?

in "the war of the worlds", the invading aliens are defeated through bacterial infections and viruses that they have no immunity to because this is their first exposure to such pathogens. hypothetically, if aliens were to invade earth's soil, wouldn't they be exposed to the bacteria or other malicious microscopic organisms that earth inhabitants are already immune to? just curious, because no other alien movie [that i am aware of] covers this possibility. doesn't this make earth somewhat safe from any alien invasion? asking because i'm hearing a lot of hype for the "judgement day" of may 21, 2011. some people believe that an alien invasion will take place