Fall sick
Definition 1 of 1
Pro Tip 1/3Context Matters
Use when discussing health, often unexpected or due to external factors like food or weather. 
He fell sick right before the marathon, probably because of the cold weather.
Pro Tip 2/3Sudden Illness
Implies a quick onset of illness rather than a long-term condition. 
Many students fell sick suddenly during the flu outbreak.
Pro Tip 3/3Not Always Serious
Can refer to minor illnesses, doesn't always mean a severe or life-threatening condition. 
I might fall sick with a cold if I don't dry my hair.