A chimera is essentially a single organism that’s made up of cells from two or more “individuals”—that is, it contains two sets of DNA, with the code to make two separate organisms.
One way that chimeras can happen naturally in humans is that a fetus can absorb its twin. This can occur with fraternal twins, if one embryo dies very early in pregnancy, and some of its cells are “absorbed” by the other twin. The remaining fetus will have two sets of cells, its own original set, plus the one from its twin.
A person can also be a chimera if they undergo a bone marrow transplant. During such transplants, which can be used for example to treat leukemia, a person will have their own bone marrow destroyed and replaced with bone marrow from another person. Bone marrow contains stem cells that develop into red blood cells.
This means that a person with a bone marrow transplant will have blood cells, for the rest of their life, that are genetically identical to those of the donor, and are not genetically the same as the other cells in their own body.