英和和英

His condition shows little improvement.

be sick

Care killed the cat.

Worry is often the cause of illness.

suffer a severe illness

He is not so ill.

a nasty disease

His illness is nothing serious [to worry about].

His condition has improved considerably.

His illness is progressing.

Mosquitoes spread disease.

under [on] the pretext of illness

be cured of a disease

allege illness

a deadly disease

Sometimes he gets a little better but then he has a relapse [gets worse again].

He's better one minute [day] and worse the next.

one's liability to disease

His health is failing.

on account of sickness

His health is beginning to break.

His condition changed for the better.

take ill [sick]

fall [get] sick

be taken sick

fall sick [ill]

fall [get, become] ill

pick up a bug

He is not in the best of health.

simulate illness