There are such a wide variety of clinical trials (for both healthy and unhealthy participants) that most people can find one that they can participate in. Each clinical trial has its own set of specific guidelines for participation. These are called "inclusion" criteria and "exclusion" criteria.
Inclusion criteria must be met in order to be included in the trial and exclusion criteria is anything that will disqualify a person from participating. Factors such as age, gender, disease, stage of the disease, and former treatments received are the basic criteria that are reviewed.