Attenuated: Weakened, diluted, thinned, reduced, weakened, diminished.
Now "attenuated" refers to procedures that weaken an agent of disease (a pathogen). An attenuated virus is a weakened, less vigorous virus. A vaccine against a viral disease can be made from an attenuated, less virulent strain of the virus, a virus capable of stimulating an immune response and creating immunity but not causing illness.
The word "attenuated" derives from a combination of the Latin prefix "ad-" meaning "to" or "toward" and "tenuis" meaning "thin."