A disease-free plant will be healthier and more productive. There is no guarantee that a healthy-looking tree is disease free but one that seems vibrant, green, and thriving is more likely to be healthy than not.  ADAI takes steps to limit the likelihood that infected trees are imported into the state.  You should ask the vendor if their citrus trees were initially grown in a facility that protected it from exposure to citrus pest and diseases, if the citrus trees came from a nursery in a quarantine area and whether they are following the federal, State, and local citrus quarantines.