There is a lot of IMHO misleading information about this, especially cross contamination and in some cases ill-informed concern thinking you have a problem when in fact you don’t.
If people like Carl Zeiss Germany, refuse to take infected lenses, I don’t care what others say about it not being a problem. Not only can your lenses get this fungus but it can infect SLR prisms as well. If it is growing in one item, it will more than likely migrate. Living for a long time in Oman with its summers of high heat & humidity, I was very careful with my photography equipment.
Please note that fungus spores are present everywhere, it’s when they start to grow and multiply that you have a problem. To quote Zeiss “Lens surfaces are irreparably damaged by metabolic products of the fungus (e.g. acids). Its damage ranges from cloudiness to opacity caused by the film”.
Avoiding the issue in the first place is the best solution:-
Never buy used equipment from a non reputable seller.
Always check used equipment thoroughly before parting with hard-earned cash, or have a money back guarantee.
Never put lens or camera away if used in a damp environment (high humidity or rain) before making sure they are dry and then put away with a small sachet of silica gel (remember to make sure the silica gel is either new or reactivated) done by placing on a radiator or in a warm oven for a few hours. This will drive out absorbed moisture and the gel should be good for a couple of months, the process can then be repeated. How many people throw away that small sachet that comes with new optical or electronic equipment they buy – it’s silica gel and can be kept and reused for the same purpose the manufacturer placed it in the box originally.
Never use canned air, it drives dust and contaminants into areas of the lens that would need a full strip down for cleaning. Use a good quality ‘Rocket type hand blower’ and always use either lens tissue or micro-fibre cloths as anything else will eventually scratch or wear the coating away.
Make sure stored lenses and cameras are exposed to sunlight on a regular basis as dark warm moist camera bags are a perfect environment for the promotion of fungus; the spores hate sunlight and die.
I think most people will either not be aware of this fungus problem or lucky enough to live in an environment that does not cause a problem. But acquiring used equipment means some vigilance is needed.
How to check used equipment:-
Use a bright light or torch and open the lens aperture to its widest ‘f’ stop and look through the lens while pointing each end towards the light so that you can see through the lens at a slight angle – never point at the sun!. Don’t be worried if you see a small amount of dust inside, all lenses get it and unless very bad, does not degrade the image. Some lenses will have very small bubbles in the glass, this also is not a problem. What you are looking for is small fussy white patches or in bad cases, an overall cloudiness. Ken Rockwell has a very good article about lens problems (link) and I urge people to look at it because it takes a lot of experience to diagnose what is major from the mostly harmless.
While on the subject of used lenses, another worry is scratches or chips on the front or rear elements.
A chip or scratch on the front of a lens element, especially if light or near the edge, will not be a real problem unless you are taking very sharp and/or closely detailed images. But avoid buying any with them on the rear elements, as this in most cases will degrade the image; it’s better to wait for another lens to come along in your price bracket.
SLR prisms on older cameras can have their problems as well: either fungus or degraded silvering. This is harder to detect without a strip down of the camera, but a quick check by taking any lens off after cleaning the eyepiece, then opening the shutter on bulb and looking through the eyepiece will usually show any problems, it will either be hazy (not blurred, that’s the focusing screen without a lens) or show multiple dark spots. Avoid or if only slight get some TLC for your camera.