When do nine to twelve month old babies typically get sick?

Seldomly: A baby is seldom ill in the first few months of life. Much of mother's immunity transfers to the baby providing temporary protection that begins to wear off by three months. At this point, we begin to see fever, congestion, or diarrhea as a sign of babies new susceptibility to viruses.

Exposure: Nine to 12 month old babies are generally healthy unless they have recurrent exposures in the family (older children with viral infections) or they are in daycare with common exposures. These illnesses are typically brief and resolve with supportive care. Children who are breastfeeding continue to receive some passive immunity from breast milk, although they can still get sick from exposures.