Immune system
The human Immune System is influenced by several factors on Mars. A Mars colony would live in an artificial environment with a different microbial composition. Many microbes might not have made the trip. Other germs may behave differently on Mars. Low gravity and other factors may make the human physiology susceptible to unforeseen ailments.
There is evidence that many people develop allergies if growing up without frequent contact to other children, or exposed to lesser illnesses (such as chicken pox). Radiation may lower the resistance of the immune system. although there are studies (reference) that show that a low level of radiation might be beneficial to some.
