
Virulence
Disease Severity
Use 'virulence' to describe how aggressive an illness is, focusing on its capability to cause damage. 
Not Contagiousness
Don't confuse 'virulence' with how easily a disease spreads, which is 'contagiousness' or 'infectivity'. 
Objective Measure
'Virulence' is often used in a technical or academic sense to express a quantitative measure of harm. 