adj. diagnostic beyond any doubt for a particular disease.
the GNU version of the Collaborative International Dictionary of English
adj. Specially or decisively characteristic of a disease; indicating with certainty a disease.
The Century Dictionary and Cyclopedia
In medicine, indicating that by which a disease may be certainly known; hence, belonging to or inseparable from a disease, being found in it and in no other; characteristic: as, pathognomonic symptoms.
Word Usage
"Descriptions of this virus is pathognomonic, or diagnostic, of a virus that came from Robertsons circle of friends, Dr. Horowitz charges."