Yes, it is quite common for them to fall sick once they enter infant care. Their immunity is still weak and unfortunately being confined in an environment with other children that are potentially sick (like runny nose, flu, cough) exposes them to the same virus.
Even my baby that is not going to infant care gets her fair share of runny nose and cough from her 2 school going brothers. It is hard to avoid as we can't just separate them everyday. Alot of the viruses are also air borne.
What you can consider doing is to feed your LO some supplements to boost his immunity.
Childlife have a first defense supplement that can be consumed from 6 months onwards :-
http://sg.iherb.com/ChildLife-First-Defense-4-fl-oz-118-5-ml/6367
Otherwise, Sambucol is also another good option, however please note there are 2 versions - US and UK. The US version is for 2 above, the UK version is for 1 and above. So please make sure you buy the correct one. I buy mine online direct from UK here
http://www.echemist.co.uk/product/sambucol-black-elderberry-extract-formula-for-children
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