Three teenagers were arrested after an elderly couple were allegedly coughed at in the street.
The couple were approached by three people in Hitchin, one of whom is said to have coughed in their faces.
A passer-by intervened and there was "an altercation" which left a woman in her 70s with a black eye, police said.
Three males aged 16, 18 and 19 were questioned on suspicion of actual bodily harm, affray and criminal damage after the incident on Friday afternoon.
Hertfordshire Police said the elderly woman was taken to hospital for a check-up and later discharged.
Her vehicle was also damaged in the incident in Paynes Park.
The man who intervened also suffered bruising, police said.
The suspects were questioned and released under investigation.
The force called for anyone with information to contact them and asked people not to post details of those they believe are involved or share footage on social media as it could hamper the investigation.