I saw a mirror and a metal plate. Both of them looks dirty but they don't seems to be anything growing on it although the metal plate has a few dirt(??) on it. Worried because someone lend me his 18-55 and I realised there is fungus in it.
You worry too much,even if there is fungus in the camera mirror box it's easy to clean.Get a big blower brush to brush off bigger pieces of grit and dirt and then blow off lighter particles
before any chemical cleaning that way you minimise scratching mirror or lens element.
Next get a bottle of alcohol/IPA (isoprophyl alcohol) from cosmetics section of dept. store or pharmacy or methylated spirit (fire wine)from neighbourhood hardware or paint shop.DO NOT BUY THINNER.Some cotton buds too and soft tissue paper for final wipe off.Some may not want to use tissue
that's your choice.
You should check/clean mirror and viewfinder after every time you bring camera out for a shoot.
Fungus takes longer time to form than most people think so regular inspection will minimise fungus
forming.It's important to have a clean mirror because the metering sensor reads of the image reflected off the mirror in the penta mirror for low end models to a silvered solid glass pentaprism
in high end models which translate to meter accuracy and focusing. Also bright image through the viewfinder.
How does fungus look like? It starts as a dot and grow bigger like a spider web (spider will probably be drunk as the web will be irregular),imagine ice cracks
you'll get the picture.If it gets to this stage there is the possibility that the fungus have eaten
into the lens coating.Whether it affects the image depends on location and extent of spreading of fungus.You can still take photos and the image is still usable unless you enlarge them then it may be obvious.