Well, surviving an infectious disease might be affected by our genetics so we try, through public health, to avoid that form of evolutionary selection. Evolution doesn’t aim to reduce population size though – it works by favouring the fittest.
Another bit of modern medicine is affect our evolution: the regular use of Caesarean sections.
“More mothers now need surgery to deliver a baby due to their narrow pelvis size, according to a study.
Researchers estimate cases where the baby cannot fit down the birth canal have increased from 30 in 1,000 in the 1960s to 36 in 1,000 births today.” http://www.bbc.co.uk/news/science-environment-38210837