fall ill相对应的延续性动词是be ill。
fall ill是瞬间性动词，不可以延续，所以不能和since短语连用。
如：Be careful not to fall ill．注意不要生病了。It is easy for the old to fall ill in winter.老年人很容易在冬天生病
be ill是延续性状态，所以可以跟since连用。
比如：She has been ill since she came home from the rain at the seaside yesterday。自从昨天在海边淋雨之后，回家她就生病了。
She doesn't want to eat anything because she has been ill for a while。因为她这段时间生病了，所以什么东西都不想吃。
Since his illness he has been unable to do anything but lie in bed。自从他生病之后很多事情都做不了了，只能在床上躺着
fall ill.生病，属于非延续性动词，它的延续性动词形式是be ill，例如，He has been ill for twenty days，他病了20天了.
