Bibs,
could be a number of things- can you add some further detail?
Sudden illness or come on over a few days - as in "woken up this morning" and not before?
How many out of how many are suffering?
what sort of cough - raspy, from lung or throat, wet or dry?
Any other symptoms - nose discharge,lethargy, eating/ not eating
Check water is clean and running.
Ones with symptoms all at same stage,or some worse/better than others
Have you taken temperature - if so take a second time and let us know what it is and is it going up or down
What environment are they in, how lonmg have they been there - have you moved them recently,or has something started to flower/fruit/grow
With recent rain is housing/bedding warm and dry?
What is different today than yesterday that might cause problem?