So is this avian influenza virus changing to become more transmissible? How concerned should we be?
Here's what we know so far.
The latest person who tested positive for bird flu is the first to show certain symptoms: cough, congestion and sore throat. The other two patients had eye infections.
The dairy worker who had direct contact with an infected cow was given antivirals and is now recovering.
"There is always a possibility that when people have respiratory illness and have symptoms, they can transmit it to other people," said Infectious Disease Specialist Dr. Suman Radhakrishna with Dignity Health California Hospital Medical Center.
She said the new H5N1 variant does not appear to be transmissible among people.
The Centers for Disease Control and Prevention has not reported any cases of human-to-human spread so far.
"As far as we know, the family members of these infected individuals do not have any symptoms," said Radhakrishna.
The variant has been detected in wild birds across all 50 states. The United States Department of Agriculture announced an ambitious $800 million program to support testing, surveillance and the development of livestock vaccines.
Meanwhile, health officials are urging frontline dairy workers to wear personal protective equipment and to get the regular flu shot to prevent contracting two illnesses at the same time.
"The flu shot will bring up the antibody levels in the farm workers," said Radhakrishna.
As for the general population, health officials say the risk remains very low. Radhakrishna said if you're planning to visit rural farms or ranches this summer, monitor CDC warnings for avian flu activity in those areas and avoid raw milk.
"We are recommending that you consume all pasteurized products only, for now, while we wait and get a control on this infection," Radhakrishna said.
The USDA says the commercial milk supply remains safe.
As part of its preparedness efforts, the CDC is adding about 5 million doses of bird flu vaccine to the national stockpile in case it becomes necessary.