I never knew. Or, I feel like I've always subconsciously known that jeans weren't the best clothing item, but I never really thought about it because THEY'RE EVERYWHERE.
This link will provide the more insight than I can, but I'll give you a brief overview.
The most apparent problem is the dye within the jeans. It's turning beautiful, natural rivers into blue nightmares as well as polluting the farmland along the banks.
Look into a brand before you purchase.
Only wash them when necessary; it is not unsanitary to wear them more than once between cycles!