Irena Ivic, a driver for the Milwaukee County Transit System, was stunned when she saw a young child running barefoot down a highway overpass on the morning of December 22.
The child fell asleep in Ms Ivic's arms as she waited for emergency services to arrive.
Another passenger brought her jacket up to the front of the bus to help the boy warm up.
"Oh my God, I was so upset", she told KTLA sister station WITI in Milwaukee.
A passenger on the bus took off her winter coat and draped it around the baby girl, who was cold to the touch. The boy was chilled after spending time outside in the freezing temperatures.More news: Amazon's IMDb Launches Free Movie and TV Streaming Service
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Ivic sat talking to the coat-swaddled baby, stroking her hair.
According to authorities, the toddler was discovered to be less than a year old and it's believed the mother, who may have been suffering from mental health issues, may have left the boy outside.
"Oh my God, I'm shaking", Ivic can be heard saying when she returns to the bus with the girl.
Authorities reunited the baby boy with his father.
This is the ninth lost or missing child who has been found by MCTS drivers in recent years.
NOTE: Police first said the baby was a boy but later clarified that the child is actually a girl.