The BBC Points West presenter spent a week in intensive care after picking up the disease.
Mr Garmston said he thought he was suffering from anxiety at travelling post pandemic - but was also coughing and struggling to breathe.
Several Covid tests came back negative - and his family called an ambulance when he passed out on his bed.
He was diagnosed with legionnaires’ disease at Bristol Royal Infirmary.
The 63-year-old said: “It turned out that I was incubating this rather nasty disease.
I’m deeply touched by all your lovely messages from viewers/friends More than I deserve. Heartfelt thanks to everyone at the Bristol Royal Infirmary, especially the nurses in Intensive Care who held my hand day and night. And to the wonderful tea ladies who brightened my days.
— David Garmston (@DavidGarmston) September 21, 2021
“The next thing I remember an ambulance arrived, although I can barely remember that, and then I woke up in hospital.”
Mr Garmston paid tribute to all those who had cared for him.
“I was very fortunate they were on top of it. They found what was wrong quickly and gave me the medication to specifically target it,” he said to the BBC.
“The staff were amazing, particularly the nurses in intensive care who held my hand and never let me out of their sight,” he added.
Fellow Points West presenter Alex Lovell said the team had been inundated with viewer emails concerned about Mr Garmston.
“We’ve have been increasingly inundated with emails. Viewers are clearly concerned at the absence of someone who’s been part of their local news family for years,” she said.
“He’s clearly so loved and we miss him hugely at work. We’re just delighted he’s getting his strength back,” she added.
Mr Garmston is now preparing to return to work.