The **Exercise in Child Health** study is a project funded by the NIH, working with several major hospitals. It aims to understand how exercise affects children with chronic diseases like cystic fibrosis (CF), sickle cell disease (SCD), and those affected by SARS-CoV-2. These are serious conditions, but today's medical advances allow children to live longer. It's important to find out how much exercise is safe and beneficial for them. **Key Points:** - The study lasts 3-4 years and involves annual exercise tests. - Participants will be children aged 10-17 who have CF, SCD, or were affected by SARS-CoV-2. - The goal is to create better ways to measure fitness and activity for these children. The study checks how exercise affects things like inflammation and growth. This will help doctors understand how to improve the health and care of children with these conditions. Participants must be in good health for their condition and able to exercise. If you think you or your child might fit, consult with your doctor to see if participating is right for you.