This is a popular question throughout this site as well as the internet in general. However the same answers which address storage are used for usage. In this question I am specifically focusing on the active usage. That is, how do you avoid "catching" fungus while actively shooting in a warm humid environment? And if you do catch it, how do you remove it preventing re-occurrence?
The two most prevalent answers involve: Silica Gel and UV/Sunlight. Both of these I would like to discard as not applicable for active usage, that is, when you're constantly taking the camera out of the bag and placing it back.
fungicidal pellets, ammonia, borax or some other chemicals. However their safety, usage, dosage, or combination are not clear.
Camera bag composition
This I haven't seen discussed much anywhere at all actually. From my personal experience, however, I can confirm that fungi are not omnivorous and do prefer certain plastics and materials. However I can't confirm composition, only that some are more susceptible than others and presumably that some are immune. Goes without saying that if you put your camera in a bag that has developed fungus there's very high likelihood of it spreading to your gear.
Some background about my particular case
Fungus is relatively easy to "clean" with just light soap and a cloth. I even added a very light ammonia to the mix (too light perhaps?). I've done it, and it was gone for a year. However, it returned the next year (now), coinciding with the cycles of tropical environment of the country where I'm presently staying.
Initially all my 3 lenses and even the camera sensor were affected. The sensor was completely covered when I noticed the problem with the lenses. (The humidity at that particular time seemed most severe.) Although the sensor is still clean now, all 3 lenses got the fungus like a clock at the same time again. I usually pay attention to equipment, so I would estimate that it went from unnoticeable to what you see in the image I attached, in about a week or less.
So a real solution would involve something that keeps it away, or kills it permanently, when you happen to catch it. So if you do get it again, it comes from a different source. Clearly in my case I didn't kill it thoroughly. Taking electronic lenses apart is very hard, with a chance of damaging the fragile ribbon cables inside, so goes without saying - best minimized.
In this section I describe in detail why Silica Gel and UV/Sunlight are not applicable to this particular question. You can skip it, as long as you keep it in mind.
Silica Gel as a preventive measure
Silica Gel (and similar moisture absorbing chemicals) are very effective when it comes to storage of photo gear. For camera shops or people who wish to store their gear it is a perfect solution. However, in this question I address a situation where a person is actively shooting in a warm, humid environment and for such scenario silica gel is completely useless. The reason being, as you frequently open your camera bag the gel is absorbing the humidity around you rather than from the camera.
UV/Sunlight as a way to kill the fungus without opening the lens
No matter how long and what kind of light you shine at the fungus infected lens, fungus will not fall off. It will just stay there, degrading the image quality. No matter the duration or the type of light you use, there's no guarantee that some spores are left unexposed. And since you leave fungus remain on the lens it can not be distinguished from dead or living. So this is very risky procedure. However, I should mention, there are some people which will chose to leave the fungus on their lens in order to avoid servicing or disassembling it (or for whichever other reasons). For the sake of this question, I am not addressing that scenario, and not considering it a solution simply because it doesn't solve the problem. Visually, fungus is still there. Is it alive? It's unknown. However, for storage-related questions, it is a good practice to occasionally take the lens out of storage and dry it in ventilated area exposing it to sunlight. Unless if you're in humid environment. Then it's better to just take it out of storage and swap silica gels with fresh ones.
Caution: Exposure to direct sunlight has a very high chance of damaging the electronics inside the lens. Also, as the sun moves, an angle could shift causing a concentrated beam which could possibly damage something as well.
- Fungus is constantly in the air around us. Yet I know many photographers who actually live in these regions and do not even use silica gells. So this would indicate that the effect of contagiousness is more important than matching the right enviromnet (humidity + warmth). So this would suggest that if you clean everything thoroughly, you should not get reoccurence. For example, my sensor is still clean, even though the lens attached to it has fungus on inside. If it were on the outside of its bottom I suppose the sensor would easily get infected at least a little bit. But the fungus most likely "activated" from within the lens.
- One more thing I've read just now, about storing lenses in ziplock bags with some silica gels. BAD idea. There's air inside the bag, and if you're in a hot environment this air can convert into condensation (moisture) which is bad for electronics inside the lens. But even aside from that, moisture is fungus's best friend.