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 immune system. 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 and sickness like a common cold can become deadly. Unfortunately, SCID patients don't live to see their first birthday if they fail to receive treatment.

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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. It's a non specific immune response.

On the other hand, adaptive immune cells are like special forces that are adaptive at handling more complex situations, enabled by specific immune response. 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. This is more commonly known as antibody.

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