Miles McCann-Robinson contracted bacterial meningitis, putting people he had seen in the previous week at risk.
"School was out; we were overwhelmed by just the fact he was in ICU, and now we had to search and try to find all these people he may have infected," Miles' mother, Nadya McCann, said.
Mccann went to her laptop and logged onto Nextdoor.com, an online social network designed to link neighbors in close proximity.
Miles' case was a health emergency, but Nextdoor is more commonly used to get neighbors to share tips on finding a babysitter or a gardener or to discuss community issues.
Redwood City has 28 Nextdoor neighborhood groups. The city is enthusiastic about it.
"Nextdoor provides the opportunity for neighbors to get to know neighbors through a social network but a very localized, small geographic based social network," Redwood City Communications Manager Malcolm Smith said.
It also addresses a growing problem.
"I may know my neighbors on my left and right, but I have no idea who's living across from me, down the street a bit at all anymore," Belmont resident Glen Morgan said.
Nextdoor networks have been created in 974 neighborhoods in 43 states. It takes 10 households to start a group.
"It's not like a Facebook or a Twitter where you're talking to people that you'll never meet in person, it's about strengthening and amplifying real-world relationships that already exist," Nextdoor co-founder and CEO Nirav Tolia said.
Nextdoor opens dialogue that can even save lives.
"I was like grateful all my friends, their parents, get the message so they could take the antibiotics because a lot of them like could be dead right now," Miles said.
The number of neighbors being brought together by Nextdoor is growing with 10 new neighborhoods added every day.