Unfortunately biting is quite common amongst toddlers and a phase they normally grow out of as they become able to express their emotions through language and in other different ways. Your nursery should have a policy of dealing with biting incidents, normally involving shadowing the 'biter' and removing them from a situation, hopefully in time. However having had a biter myself I know how extremely difficult it can be to prevent even if all these things are done and you are monitoring a child closely. My son would be happily playing and very well behaved and the next split second had gone and bitten another child (often his sister).
If you are concerned talk to your nursery and they can explain their procedures to reassure you what steps they are taking to deal with it.