The number of cases and severity of allergies in humans has been on the rise in recent decades, leading some people to think there is a link with modern lifestyles. However, no single theory for explaining allergy increases has yet to be fully established. One idea is that children are not exposed to enough bacteria when young, so their immune systems don't 'learn' when to react and when not to. However, this does not explain cases of allergies in very young children and babies. Some people think it is linked to the prevalence of chronic diseases, such as morbid obesity, in wider society.