That depends a lot on the species. Some species require higher humidity in general and if the humidity is low, and they are already fighting an infection it could be a problem. Others need a very dry environament and high humidity could cause them problems. In general, I think increasing temps a little is more beneficial in stimulating the immune system than increasing humidity, especially if you are administering meds because the temperature has a direct effect on metabolic rate.
:When a snake has a respitory problem or is "sneezing", if you increase the humidity will the snake get better after a short period.