We all know about how the aliens in War of the Worlds were killed by the common cold, but how susceptible would aliens really be to human diseases?
In my story, I have an alien species living among humans. The aliens are able to eat most of the same foods as the humans and have been on the planet for around six thousand years. Would they likely catch human diseases? Or would they be immune to them because the diseases would have to mutate immensely to affect them?
Viruses depend on our cells to reproduce. They "inject" their genetic code (RNA/DNA) into our cells and force them to create new virus cells which then burst out of the host cell. Some types of viruses depend largely on the host cells organelles while mostly larger ones do some of the brunt work themselves.
The key factor to virus reproduction is the availability of amino acids of which they are constructed. If said alien's cells do not contain amino acids then they would not be susceptible to viruses as they cannot reproduce. So Paul is unlikely to get AIDS.
As for bacteria, I really cannot say. They usually produce toxins and this is what the body reacts to. They can grow wherever there is food, a nice temperature and humidity. There are even ones that photosynthesize their own food. You would have to decide yourself whether your aliens provide a good base for bacteria to grow on and if they are susceptible to the toxins.
Depending on how different they are, most bacteria won't affect them in the beginning. There might be one or two that cause some surprising affects but most I think would find them too 'alien' to deal with. What ever attacks them would be something likely we'd find innocuous.
Some parasites can live in the human body from other species and some can't.
After 6000 years though, I would expect some of the bacteria has found it's way to sicken the aliens, so even if they were different enough to not be affected at the beginning (at least from any native diseases) After 6000 years there is a good chance something would start to go after them. The worst of course would be ones that have a two stage life cycle, needing both an alien host and a human host to complete. There are many parasites that do this. Swimmers itch is one such, (water fowl and snails)
They've been here 6000 years. So they can obviously cope with Earthly diseases in some way or other.
I'm more interested in your statement "The aliens are able to eat most of the same foods as the humans". MOST? Which foods CAN'T they eat? That seems a promising path to explore...
("Affect" not "Effect" BTW. Seeing as you're writing a story, you might as well learn English :-)
Considering the alien race to be just another species lurking on the face of the earth, only this one's ancestors don't trace back to the same roots, one could just as easily ask if they were suspectible to diseases carried by parrots.
It's unclear from your question how the aliens and humans are able to coexist. Is it because they are indistinguishable from humans, or because they are actually cattle and we just don't know they actually came from the planet Cowanus, or do they simply live amongst us as an acknowledged party of society? In any case, I think it's safe to assume that these aliens are unable to reproduce with humans, as they do not share... well... anything.
At the moment of arrival on earth, many of these "alien pioneers" probably did die to numerous diseases (viruses, bacteria, parasites, fungus...), but as always, a small amount of them survived. These aliens have, in the past 6000 years, evolved into a species that is accustomed with Earth's mother nature.
Another way to look at it is that they had the technology to come to this planet, from assumably very far away. They are hence likely to pack a bunch of medical resources as well, so just dying from everything that touches you is a bit much. Of course, this doesn't mean they aren't "suspectible" to these diseases, just that they're not immediately kicking the bucket.
So to answer you question, sure, they would be suspectible to human diseases. They would be equally suspectible to diseases carried by butterflies, gorillas and Lucius Malfoy.
