Attenuated: Weakened, diluted, thinned, reduced, weakened, diminished.
The use of "attenuated" in medicine is not new. In the 16th century, eating dried figs was claimed to attenuate the body fluids.
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."