If a baby displays signs of possible illness, and the circumcision might be dangerous, the baby is not circumcised at the appointed time. As an example of this, the Gemara relates a number of stories about Rabbi Natan’s ruling in such cases:
Rabbi Natan said: On one occasion, I went to the coastal cities, and one woman came before me who circumcised her first son and he died,and she circumcised her second son and he died, and since she feared circumcising the third due to concern that he might die as well, she brought him before me. I saw that he was red. I said to her: Wait until his blood is absorbed into him. She waited until his blood was absorbed into him and then circumcised him, and he lived. And they would call him Natan the Babylonian after my name.
Rabbi Natan further related: On another occasion I went to the state of Cappadocia, and a woman came before mewho circumcised her first son and he died, and she circumcised her second son and he died. Since she feared circumcising the third due to concern that he might die as well, she brought him before me. I saw that he was pale. I looked at him and I could not see in him the blood of the covenant, i.e., he had a blood deficiency. I said to her: Wait until blood enters him. And she waited and then circumcised him, and he lived. And they would call his name Natan the Babylonian after my name.
Several medical explanations were proposed to identify the diseases from which the babies seen by Rabbi Natan were suffering. With regard to the child who was red, some presume that the children of that family suffered from a rare hereditary blood disease known as purple disease, purpura hemorrhagica. The child who was pale was probably sick with hemolytic jaundice of the newborn, which is also hereditary.