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There's actually a window before fungus becomes a permanent fixture - if you catch it early and the lens still has good optics otherwise, storing it in a sealed container with silica gel and leaving it in a warm, dry place for weeks can sometimes slow or halt the spread, buying you time to decide if professional cleaning is worth the cost. The trick that doesn't get mentioned much is using a dehumidifier in the room where you store your gear; fungus thrives in humidity above 60%, so keeping your storage area consistently dry (ideally 30-50% humidity) prevents new growth and can actually reverse very light surface fungus on the exterior elements. If the fungus is already deep inside the lens elements themselves though, you're right that disassembly is the only real fix, but preventative storage is way cheaper than paying for a professional teardown.
Fungus inside a lens isn't really something you can fix yourself - it's not like cleaning the outside glass. Once it's gotten into the sealed elements, you're looking at a disassembly job that needs proper equipment and a clean room environment. Trying to open it up at home will just introduce more dust and make things worse. Your realistic options are sending it to a repair shop or replacing the lens, depending on how bad it is and what the lens is worth.
Before you go that route though, make sure you're actually dealing with fungus and not just dust or haze on the exterior. A lot of people think they have fungus when it's just surface grime. Clean the outer lens elements thoroughly with proper lens cleaning supplies - microfiber cloth and lens cleaner or even just breath and a soft cloth. Look at it against bright light and see if the spots or cloudiness actually disappear when the outside is clean.
If it really is internal fungus, get in touch with a camera repair shop. They'll open the lens in a controlled environment, clean the affected elements carefully, and reassemble it properly. It'll cost you, but it's the only way that actually works. The price depends on your lens and how extensive the fungus is, so get a quote before committing.
The thing is, once fungus is actually inside the sealed lens elements, you pretty much can't reverse it yourself - it'll keep spreading and eating away at the glass coatings no matter what you do with silica gel or storage tricks 😕 Your best bet is sending it to a repair shop that can open it up and clean the elements properly, though depending on the lens value that might not be worth it.
If you're dealing with fungus that's actually visible inside the lens elements, professional disassembly is your only real option - but here's the thing nobody mentions: how fast it spreads depends a lot on humidity and temperature. If you catch it early when it's just starting to cloud a small area, a good repair shop might be able to clean and re-coat the elements before it damages the glass permanently. The catch is that once fungus has been eating away at coatings for months, even professionals can't fully restore optical quality, so getting it serviced sooner rather than later makes a huge difference in whether it's worth fixing versus just replacing the lens.
When I dealt with this at home, I found that catching it early matters - if it's just starting to show as spots on the inner glass, keeping the lens bone dry in a sealed bag with fresh silica gel for a few weeks can sometimes arrest the spread, though it won't erase what's already there. Past that point, you really do need someone to crack the lens open and clean the elements properly, which isn't cheap but beats replacing the whole thing.
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