At Descanso Pediatrics in La Canada, phones are ringing non-stop and flu tests are yielding about a dozen positive results every day.
"Flu, bronchiolitis, colds, ear infections," said pediatric Dr. Neville Anderson.
Seven-month-old Roman Juarez has a 2-year-old brother at home who was sick with the flu. Now Roman's feeling the effects.
"He's had a runny nose and chest congestion. He's been crying, keeping mommy up all night," said Roman's mother, Katherine Juarez.
Despite her best efforts, Katherine couldn't keep Roman from catching his brother's germs.
"The other one wants to hug him, wants to kiss him. I wash their toys every day," said Katherine. "We wash our hands, take showers twice a day. I don't even know what else to do."
Doctors say washing hands is a very important part of the strategy to keep siblings safe.
The second is to get a flu shot. Katherine got hers earlier in the season, so even though she's feeling a little under the weather, doctors say it could've been worse.
"The flu shot this year seems to be about 60-percent effective. So if you get the flu shot, it's still a chance you can get it. It tends to be a milder illness than if you didn't get the flu vaccine," said Dr. Anderson.
Roman got his flu shot a month ago, which doctors say might explain why he didn't get a high fever.
As for keeping Roman away from touching germy items -- well, that can be a challenge. Parents have to pay attention.
Many infectious disease experts say what we are seeing now is a ramp-up in flu activity. The worst wave should be hitting California in the next two to three weeks. So it's still a good time to get a flu shot since flu season can go as late as April.