A child born with severe combined immunodeficiency (SCID), does not have an immune system that works properly. The immune system is made up of cells in the body that protect a person from sickness. A child with SCID doesn’t have that protection. Illnesses like the flu or even the common cold can make a child with SCID extremely sick and could even be fatal.

There are about 20 different defects in the genes that can cause SCID. Each one of these genetic defects is considered a type of SCID. Most types of SCID are considered typical SCID, characterized by low or no T cells and a non-functional immune system.