virulence

vir·u·lent

adj.

1.

a. Extremely infectious, malignant, or poisonous. Used of a disease or toxin.

b. Capable of causing disease by breaking down protective mechanisms of the host. Used of a pathogen.

2. Bitterly hostile or antagonistic; hateful: virulent criticism.

3. Intensely irritating, obnoxious, or harsh.

virulence

n

1. the quality of being virulent

2. (Medicine / Pathology) the capacity of a microorganism for causing disease

But the oral vaccine has its drawbacks also. In rare cases, the live but attenuated viral strains in the vaccine can themselves cause polio.

Moreover, these viruses can revert to virulence, resulting in what are known as vaccine-derived polioviruses (VDPV)