Extremely virulent strains can eventually Evolution by mutation and natural selection within the virus population inside a host.

"It is of an extremely virulent nature, and all the cases that we have seen to date have exhibited rapid deterioration."

A tall, extremely virulent, and unusually powerful pathogenic agent.

They thought it was an extremely virulent strain of the flu.

It has been hypothesised that an extremely virulent pathogen might not evolve.

Small and aggressive, carpet vipers have an extremely virulent hemotoxic venom.

The strain is extremely virulent and may have erased it all.

All three are extremely virulent and produce the same disease symptoms.

We are facing an extremely virulent strain and the environmental conditions lead us to fear the worst.

According to the accounts of the time the Black Death was extremely virulent, unlike the 19th and early 20th century bubonic plague.