Fungus spores are tiny and are everywhere. They are distributed in the air. You can assume that everything is infected by fungus. But they are just tiny spores. The trick is preventing the spores from growing. That way they stay microscopic and cause no trouble.
Keeping fungus from growing is as simple as keeping the humidity low. I don't know the exact humidity threshold, but have heard 70% relative humidity mentioned. Stay below that. Fungus also needs time to grow, and I have heard it needs a couple of days. Fungus also loves the dark.
So don't store your gear at higher than approx 70% RH for longer than a couple days in the dark.
Most places where I live and travel have lower humidity than that, because they are heated in the winter and air conditioned in the summer. There are exceptions, of course. There was a risk in the Amazon where it was very humid and no AC. The lodge provided a cabinet with a light in it, intended to create some warmth. That reduces the RH. I managed there for 5 days without any other measures and it was OK.
One time in the winter I put my camera bag away after a shoot, not realizing there was snow clinging to the bottom. A week later I went to get the camera, and everything inside the bag was covered in fungus filaments.
The snow had melted and soaked the bag. My 28mm lens was pretty bad and needed repair. The other gear was only affected on the outside.
Fungus needs nutrients to grow. The filaments have evolved the nasty property of etching glass surfaces to get the minerals it needs. So even if you remove all the fungus, a glass surface might still be etched.
So you definitely don't want to store your gear in conditions where fungus thrives. Pay attention to the humidity and don't let your stuff sit around long if it is wet.