She has just recovered from a severe illness.

她大病初愈。

《新英汉大辞典》

However, some people with swine flu get more severe illness.

然而，某些人感染猪流感会得很严重疾病。

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Hepatitis a is a viral liver disease that can cause mild to severe illness.

甲型肝炎是一种病毒性肝脏疾病，它可造成轻度或严重疾患。

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Elyn Saks is a legal expert who works to expand the rights of people with severe mental illness.

VOA: special.2009.10.02

He was placed in a mental hospital and told he had a severe mental illness.

VOA: special.2010.04.14

A severe respiratory illness was affecting young people, contrary to seasonal viruses that often attack the elderly.

VOA: standard.2009.12.25

They tend to have a less severe illness (although it can still be debilitating).

BBC: Whooping cough makes a comeback

It is not necessarily a precursor to Alzheimer's disease, but it does raise the risk of progressing into that more severe illness.

CNN: Alzheimer's: Early detection, risk factors are crucial

The strain H3N2 of the flu virus is causing many of the cases, reports suggest, and it is known to cause severe illness, especially among the elderly.

BBC: Most of US gripped by flu outbreak