When do six to nine 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.

Illness 6-9 mos: Babies under one year are more vulnerable and can get ill more easily. Observe in daycare and large crowds. Good hand washing, head elevation, fluids and Tylenol if pain, fever.