The outbreak has followed the All Blacks on their end of year tour with four All Blacks, one by one, taken down in as many weeks.
Jack Goodhue, Rieko Ioane and Ardie Savea all contracted the infectious viral disease, and now lock Luke Romano.
“Luke Romano's in semi-quarantine, because he's got them as well, so I think that might be four cases,” says Coach Steve Hansen. “And all of them have been vaccinated.”
It follows a huge spike in mumps cases in Auckland this year - now nearing 900 - numbers not seen in decades.
So what's causing it? Firstly, it's low vaccine coverage.
“There's a lost generation, from age 11 through to 30, who have very low vaccine coverage,” says Dr Helen Petousis-Harris, senior lecturer at Auckland University and director of research at the Immunisation Advisory Centre.
“Some parts of the community, you're seeing maybe 30, 40, 50, 60 percent in some pockets, who haven't received the vaccine.”
She says some Pacific Islands haven't used mumps vaccines in the past and some people think they're fully vaccinated, when they're not.
On top of that, Dr Petousis-Harris says the mumps component of the MMR vaccine is only 86 percent effective - and it wears off.
“It's sort of in the 10-15 years [after immunisation] that you start to see more cases in the vaccinated, so when you put all of that together, you've got a perfect storm for an outbreak. So, once you've got the outbreak of a highly infectious disease and you put people close together, it can go whoosh.”
Mumps is easily spread through the air or contact. Symptoms include:
*Pain in the jaw
*Swelling of the glands around the face
Seven percent of cases have been hospitalised - that's 64 people so far this year.
Public health officials say people need to have had two MMR vaccines to be immunised. If in doubt, they recommend - and say its safe - to get another MMR shot