I'm curious, how do you think they could be harmful to our species in the long run?
Most of our evolutionary strengths were caused by viruses, getting sick made us stronger as a species. If people who do not need a vaccine do not get sick the body doesn't learn how to adapt to new symptoms. Thus eventually there will be a virus that we will be unable to make a vaccine for, when it does if enough of us are unable to survive it because of a lack of practice then our species will be wiped out. I don't really mind, living my last days without any other humans would be interesting and even nice, but I am sure many would care.
No offense, but I think you may be conflating two phenomena - the body adapting to an infection and the body 'adapting to symptoms'. Yes, our immune systems, if not compormised, will adapt to infection and that is the principle behind most vaccines. We're infected with the vaccine, but the virulence is inactive (most won't get sick). But, I'm not sure what you mean by our bodies adapting to symptoms. I'm not an MD, but I have some basic ideas in this area...enough to make me dangerous and say stupid things, if you know how that goes. So, I could be misunderstanding you.
Plus it's early in the morning...so I have that goin' for me (not).