How common is a weak immune system?

Low immunity: It is commonly called that and a lot of "stuff" is sold to boost it (notice no before and after tests to prove anything). Actually altered immunity is common and likely seen with allergic individuals. Selective deficiency of the immune system do exist but are less common. A good learning experience to help you understand this system is http://www.Nlm.Nih.Gov/medlineplus/immunesystemanddisorders.Ht.