WMG / Melanie's Marvelous Measles

Sam has actually always had the weak immune system.

Tina is established to have a brother named Sam, who got given shots, but then got "very sick" afterwards, and now he has a "weak immune system". Tina and Sam's mother thinks that the shots are what caused the weak immune system, but it could also be that he's had it from birth, and the weak immune system was what caused him to become "very sick" after being vaccinated.