An on-duty bus driver in Milwaukee knew what she had to do when she saw a young boy when a baby wandering around alone near busy streets.

Milwaukee County Transit System (MCTS) driver Irena Ivic spotted the boy, wearing only a red onesie and no shoes, running down the sidewalk at 8 a.m. on Dec. 22, 2018. Surveillance video from the bus shows Ivic's view of the boy as he ran over a freeway overpass. Another security camera captured her reaction in the driver's seat.

The temperature was below freezing and Ivic knew she had to stop her bus and save the baby boy, who was less than 1 year old, MCTS said. She scooped up the boy and carried him to the bus. Surveillance camera footage from inside the bus shows a concerned passenger waiting by the door when Ivic returned with the boy in her arms.

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"I'm shaking," the driver said.

"Oh, I am too," the passenger responded.

Another passenger brought her jacket up to the front of the bus to help the boy warm up. It's unclear how long he had been alone outside.

While Ivic waited for authorities to respond to the missing child, he fell asleep in her arms. The video shows her lightly kiss him on his head, letting him know everything was OK.

Firefighters and police soon arrived on the scene. Police said the boy's mother left him outside, according to MCTS. Officers later reunited the toddler with his father.

Ivic said she was grateful she could keep the boy safe, MCTS said in a video about the incident. Thanks to her quick thinking and warm heart, he was kept out of harm's way.