Virulent
Highly infectious or disease-causing; spreading rapidly and causing severe harm to a host organism.
Describing a pathogen, toxin, or disease that spreads quickly and causes severe illness or damage to its host. The term originates from Latin and is commonly applied to bacteria, viruses, fungi, and other infectious agents that are particularly potent or aggressive in their effects.
What this means in real life
A new flu strain proves virulent when it spreads quickly through a school, sending many students to the hospital with severe symptoms within days.
What it isn’t
Virulent does not simply mean 'present' or 'existing.' A disease can be mild yet widespread; virulence specifically refers to the severity and aggressive nature of harm caused, not merely its prevalence.
Commonly misused online
Social media often calls any widely-shared misinformation 'virulent' when they mean 'viral'—but virulent implies active harm and severity, not just rapid spread.