Virulent describes something extremely harmful, poisonous, or intense in its destructive power. The word appears most often in discussions of disease: a virulent strain of flu spreads quickly and makes people very sick, much more dangerous than an ordinary cold.

Scientists use the word to distinguish between germs that cause mild problems and those that cause serious harm. A virulent bacterium might overwhelm the body's defenses rapidly, while a less virulent one might cause only minor symptoms. During the 1918 flu pandemic, doctors noted how unusually virulent that particular strain was, affecting even healthy young adults who normally would have recovered easily.

The word also describes harsh feelings or expressions. A virulent criticism goes beyond normal disagreement to become bitter and hostile. Someone might launch a virulent attack on an opponent's character rather than debating ideas fairly. When people express virulent hatred toward a group, their feelings are intense, poisonous, and dangerous.

Whether describing a deadly disease or toxic words, virulent always suggests something that spreads harm aggressively and powerfully. The related noun is virulence, the quality of being severely harmful or malicious.