Mites ride on your clothes, your animals, your grocery bags, wherever they can hitch a ride. Their favorite outdoor living space is natural ferns and greenery. All you have to do is bump into someone who has them on their clothes, one hitches a ride, crawls off you to your plants, and you have an infestation. They reproduce asexually, so it only takes one.
All crops have their own pests,spider mites are the most common found in grow rooms. People act like they're at the same level as an STD, but there wouldn't be a million products out there to kill them if they weren't a very common problem.
Keeping others out of your grow is always a good idea. Not bringing in clones from other grows is another way to help prevent. You can use a systemic product, like the 209 as a preventative.
You know you have them when you see little white dots on your leaves. If you treat at the first sign of damage you're more than able to control and eradicate.