I have always been prone to illness, as many people are. However, when I get sick, I’m sick for a long time. And if one of my friends are sick, there’s about a 99% chance that I’ll be sick shortly after being around them. Others are much luckier and rarely ever get sick. So, why am I always catching colds?
My family always blamed my parents for sheltering me when I was younger, but what really causes a weakened immune system?
There are many external influences that contribute to this problem. Some of these are “emotional stress, physical stressors such as inadequate sleep or athletic overtraining, environmental and occupational chemical exposure, UV and other types of radiation, common viral or bacterial infections, certain drug therapies, blood transfusions and surgery.” The way we eat can also have an impact on our immune responses. Nutrient deficiencies also cause problems within an immune system. Consuming excessive amounts of fat, alcohol or refined sugars also cause problems. Another huge cause are certain diseases, such as AIDS. Others, due particularly to genetic abnormalities, also have an impact. In relation to the nutrient deficiency, your weak immune system could be your mother’s fault. If she had low nutrients while pregnant with you, it could have transferred to you and given you a lowered immune system. There are many possibilities that could cause such a thing.
If you are suffering from this, there are ways to enhance your immune system, such as consuming vitamins and supplements, exercising regularly, and stress management. Drinking plenty of water and getting a good night’s rest are also recommended. This website has a long list of things at the bottom of the page that are suggested for a diet. 
Be careful, though, because a weakened immune system can often lead to tumor growth and autoimmune diseases.