A disease, also known as illness or medical condition, was an abnormal condition or disorder to the function or structure of a lifeform that produced specific symptoms, not caused as result of a direct physical injury. Common terms to describe disease were malady, sickness, illness, or ailment. Other related terms included blight, which referred to plant disease, and plague, which described a disease that infected large numbers at once.
Someone who was exposed to a disease but was not affected by it was known as a carrier. (TOS: "Miri", et al.) Immunity to diseases could be established with the incorporation of an immunizing agent. (TOS: "The Omega Glory", et al.)
Sensors aboard Federation starships had the ability, during the mid-to-late 23rd century to reveal the presence of disease organisms. (TOS: "The Changeling")
After the crew of the USS Voyager became aware of Reginald Barclay, through his assistance with the Pathfinder Project in 2376, The Doctor reviewed his personnel file and took note of his "unusual medical history", and disregarded doctor-patient confidentiality, by telling the crew about how he "recovered from a variety of maladies, including transporter phobia and holo-addiction." (VOY: "Pathfinder")