The virus did not mutate to a more virulent form.

病毒没有变异为致命性更强的形态。

youdao

Before 1997 WNV was not considered pathogenic for birds, but at that time in Israel a more virulent strain caused the death of different bird species presenting signs of encephalitis and paralysis.

在1997年以前，西尼罗河病毒并未被认为对鸟类具有致病性，但就在那时，以色列出现了一种能够导致多种鸟类出现脑炎和麻痹症状继而死亡的毒力更强的毒株。

youdao

At present, most swine flu infections are mild, and the severity of the present pandemic does not come close to the 1918 flu, but experts worry that the new virus could become more virulent over time.

目前大部分的猪流感病例都很轻微，这次大流行的严重程度远远低于1918年流感，然而专家们担心新型流感的毒力会越来越强。

youdao

The virus did not mutate to a more virulent form.

VOA: standard.2010.01.18

Colorado's politics are not unusually virulent.

ECONOMIST: Purple Colorado: Rocky mountain coming down | The

Evidently, young Obama and his father spent hours in the company of this mentor, presumably soaking in not only his alcohol but his virulent hostility towards America.

CENTERFORSECURITYPOLICY: Characters count

It has inculcated the formerly tolerant if not pro- Israel Turkish public with virulent anti-Semitism.

CENTERFORSECURITYPOLICY: Turkey's cautionary tale