Its a good idea, could lead onto a lot more. For example, have you thought that the aliens could catch disease from humans as well. Think about the early explorers meeting up with previously unknown people and lands. It was often the case that both parties became sick because their bodies didn't contain the antibodies needed to fight off the illness. This could also lead to biological warfare. There would be a lot of new content to add to the game. Maybe someone could make a mod using these ideas.