One of the most common questions that we are asked as pediatricians is about children who are "sick all the time". Many of these children are in daycare, preschool, or grade school. Many of them are in the first few years of being exposed to common childhood infections: colds, flus, pink eye, etc. Many of these children do not naturally do a very good job of avoiding other germs: they touch the world around them then touch their face, eyes, nose, and mouth. They do not avoid playing with a toy the other 2 year old just played with and he has a runny nose and a cough. They will pick up a spoon just used by their recently ill sibling and put it in their mouth. And the vast majority of children will build up a better immunity to these germs through these early illnesses. Down the road, these children often are rarely sick as they do a better job of avoiding the illnesses and they have a better immunity to fight them off if they are exposed.
My three sons went through a similar story. Each was in daycare around many other children. Each of them went through dozens and dozens of illnesses: upper respiratory viruses, stomach flus, pink eye, ear infections, bronchiolitis, croup, roseola, hand foot and mouth disease, and fifth's disease. My oldest son was in a small, private daycare his first year. My wife and I circled the calendar date that year if he did NOT have a cold or cough that day. By the end of the year, we had only circled 5 days! Now that they are 16, 12, and 12, they are much healthier with an illness being a rare thing.
Although some children who are "sick all the time" do have an underlying problem with their immune system, most do not. And the children that do have a "immune deficiency" often have unusual bacteria cause their infections or have unusual infections (for instance, a joint infection when there was no injury to that joint before the infection). The most common immune deficiencies are self-limited -- they require no special treatment and will resolve on their own.
If you have concerns about your child and their infections, discuss it with their doctor.