SCID (Severe Combined ImmunoDeficiency) is a very rare genetic disease that affects 1 in 58,000 people. Like the name implies, it is a disease that affects patient's immunity. In particular, it affects production of T cells and B cells which are critical part of adaptive immunity. Without T or B cells, baby's immune system does not function fully. Without functioning immune system, sickness like a common cold can become deadly and patients don't live to see their first birthday. 

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Our immune system is composed of innate immunity and adaptive immunity.

Innate immune cells, such as Neutrophils and Monocytes, are like foot soldiers that are at the forefront of your body's immune system. When pathogens  enter the body, innate immune cells as the first responders. 

On the other hand, adaptive immune cells are like special forces that are adaptive at handling more  situations. When pathogens such as virus enter your body, they hijack some of your own cells so that virus can reproduce its own virus cells. This is why it sometimes take long time for you to recover from cold. In this case, the T cells are responsible for destroying the compromised cells. After the fight is over B cells remember the signature of the pathogens so that next time similar pathogens enter your body, your immune system can fight them off more efficiently. 

Babies with SCID lack the adaptive immunity in their body. Because of this, sicknesses last longer and become deadly.