Each trial needs participants who meet certain requirements, like having a certain disease or medical condition. These requirements are called inclusion criteria. Other inclusion criteria include age, sex, body type, and current treatment plan. Some trials might have rules that prevent a person from being in a trial. These rules are in place to protect people whose conditions might get worse if they were in the trial. These rules are called exclusion criteria. Inclusion and exclusion criteria are in place to make sure the researchers get the most accurate results. They also help make trials as safe as possible. Not all clinical trials test a new treatment or vaccine. "Observational" studies collect information about people's health during their normal care. This helps researchers learn more about specific health issues.