Waikato has been hit by a measles outbreak, with eight confirmed cases of the virus so far, and three potential cases.
Waikato DHB says one of the patients attended a Kapa Haka festival where 5000 people gathered and Dr Felicity Dumble says it's a particularly nasty strain of the virus.
"All of them have been in the emergency department, and of the eight cases, three of them have been sick enough to require hospitalisation."
Dr Dumble says measles is not a benign illness, and usually doctors would assume one in 10 cases will be hospitalised.
She says the first signs of measles are a fever and a runny nose, followed by a rash.