A confirmed first case of measles in Washington's Whatcom County has now produced three more, all with links to the current outbreak in B.C.'s Fraser Valley.
And the highly infectious disease, once believed to be eliminated in Canada, has also been found in Hamilton.
Health workers in Canada and in Washington State had a busy weekend keeping tabs on the measles virus and where it was spreading.
On Sunday, Dr. Greg Stern of Whatcom County, which borders B.C., said there was a confirmed case of measles and that the individual involved had been to the Fraser Valley.
The adult male showed signs of measles after returning to the U.S. and was quickly isolated. That spread the virus to three other family members. It has been reported the family attends a church in the Fraser Valley where some parishioners have contracted measles.
There have been more than 300 confirmed cases in B.C., many of them in the Fraser Valley, including a student who had been attending the British Columbia Institute of Technology campus in Burnaby, just east of Vancouver.
Fraser Health has been in contact with Washington officials and continues to explain the need for vaccination, a measure that some religious groups are against.
In Hamilton and the surrounding Halton region, media reports have said three people have been diagnosed with measles. That qualifies as an outbreak and has Ontario health workers issuing concern and additional warnings.
"Measles is highly contagious to susceptible people from the beginning of the illness until four days after the rash first appears," Dr. Hamidah Meghani, Hamilton's associate medical doctor of health, said in a news release.
The difficulty across Canada is that there are areas with a low-immunization rate, and that allows for the disease to gain a foothold before it moves across valleys and borders.