The child is healthy and his mother is recovering, the Centers for Disease Control and Prevention said. The CDC said it was virtually certain the virus came from breast milk, though there is no way to be completely sure.
Doctors stressed that breast milk is the healthiest food for babies and that mothers shouldn't quit nursing because of West Nile fears.
Last week, when the case was being investigated, the CDC urged new mothers with the virus to talk with their doctor about whether to continue breast-feeding.
West Nile is rare in infants because they spend little time outdoors and the virus is usually spread by mosquito bites. Only four children younger than 12 months have been diagnosed with West Nile since it appeared in the United States in 1999.
The 40-year-old Michigan mother gave birth Sept. 2, and received a blood transfusion that day and the next. She went home with her baby Sept. 4, only to be hospitalized on Sept. 17 for three days. Doctors later confirmed she had West Nile.
It's not clear how the mother became infected but it may have been from the blood, the CDC said. She and another patient, a liver transplant recipient, received blood from a common donor, and remaining blood samples from that donor show signs of contamination.
The government discovered last month that the virus apparently can be spread through blood transfusions, as well as organ transplants. The CDC reported 15 people this year have been diagnosed with West Nile virus within a month after receiving blood transfusions.
The virus is mostly spread through mosquito bites, and has infected 2,530 people in 32 states so far this year and killed 125, the CDC said. Most people bitten by an infected mosquito never get sick.