A note about veterinary and medical terminology.

Client:  "Yes, it's about my dog, he's got chronic diarrhoea."
Me:  "Oh, right.  How long has this been going on for?"
Client:  "Well, he was fine this morning."
Me:  *scratches head*

So, today, let's talk about words, and what they mean.  Because words mean different things to different people.  In my time at vet school, I learnt lots of new words.  Long words like 'erythematous' which I'd only ever use talking to another vet (it means 'abnormal redness of the skin' incidentally).  I also learnt that words we all use in day to day life have specific meanings in a medical context.  These meanings can be a narrowed-down form of what they're used to mean in day to day life.  But sometimes the standard usage has morphed to mean something almost opposite to the technical use.

'Chronic' then.  To my client, it means 'bad'.  To me, it means 'of long duration'.
'Acute' probably also means 'bad' to my client.  To me, it means 'of sudden (and probably recent) onset'. 
So, in techincal use, chronic and acute are more or less opposites of each other.  And my client's dog, in fact, has acute diarrhoea.

'Acute' is not the same as 'severe'.  'Severe' *does* mean 'bad'.  The opposite of 'severe' is 'mild'.  So, despite what some people thought at the time (mentionning no English-graduate journalist-types who really should have known better), SARS (Severe Acute Respiratory Syndrome) was a perfectly sensible, meaningful, and non-tautological name for the condition.  Despite the fact that in common usage 'severe' and 'acute' are used almost interchangeably.

Acute things can be either mild, or severe.  So can chronic things, though it's rarer that something is both severe and chronic, because if it's severe enough it will kill the patient before it gets the chance to be a chronic condition.  My client's dog had severe, acute diarrhoea.

I hope that clears things up a little!