Parents may conceal their children's medical problems for fear that the kids will be treated differently. The sad thing about that is that it signals to  the children that their conditions are to be ashamed of or feared. The other children in your class figure this out as well.

You should know the protocol for emergencies and acute illnesses, so make sure that the school nurse has provided you with sufficient information about the medical conditions of the children in your classroom. It's important to communicate reassurance to both the child and the class if acute illness does occur.

When one of the children in your class is absent with a communicable disease, it's important that you contact the parents of your student with medical problems immediately. Their child may need additional protection from the pre-absence exposure.

Be sensitive to when NOT to show concern, like when a child with cystic fibrosis is coughing. The cough is important to clear the lungs. Paying too much attention to a symptom often makes it worse and reinforces a child's sense of shame. 

Children with medical problems are often overly sensitive. Don't perceive their behavior as babyish or immature or a serious emotional problem. By reinforcing positive age-appropriate behavior, you are most likely to increase it.