A staff member at Waikato Hospital may have unwittingly spread measles to patients and colleagues.
The District Health Board's revealed the person was working at the hospital while they were infectious but hadn't yet developed any symptoms.
Medical Officer of Health Doctor Richard Wall says the DHB's contacted everyone who may have been exposed.
"There are nine confirmed cases. So those are ones that we’ve had lab tests back on, and then we have a further four at the moment that we have been notified about -- but we are waiting for lab tests to confirm whether or not they are measles."
Dr Wall says there was another case of measles at a regional kapa haka festival last weekend and any secondary cases from that may become evident soon.
"It is difficult to control when you have nine cases that could potentially spread further. We have one case that had attended a kapa haka festival in Hamilton. So potentially that could have led to the infection of a number of people. Those people would begin to get unwell over this weekend and the next week."