When do four to six 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.

When exposed! : This depends on many factors. Is the child immunized? Is the child breast fed or bottle fed? What are the baby's exposures? Is the child in day care? Are there siblings in school? An immunized baby in day care will be exposed to many viral illnesses, upper respiratory, gastroenteritis and viral rashes. Serious bacterial illnesses are rare in an immunized child. Hope this helps!