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 do not exist. Other germs may behave different. Low gravity and other factors may make the human physiology susceptible to unforeseen ailments.
There is evidence that many people develop alleries if growing up without frequent contact to other children, or exposed to lesser illnesses (such as chicken pox).
