I heard a story from a friend about a family that just came back from Cancun and had all these bites from sand mites on the beach. I thought it was exagerated or a fluke. But now I read another story on foders chat about La Jolla de la Mismaloya's beach having the same problem. I've only ever stayed in Nuevo Vallarta on my trips to Mexico and have never had this experience. Is this a common thing? Is it only in certain areas? Is it seasonal? Please advise.

Known as No-See-Ums they are very small flies, can go right through a mosquito net.
Always associated with still, fresh water (like the Mismaloya river), very active at dusk and dawn. They can not fly very high, bites will be mostly on your legs.
Repellent works well.

No-see-ums (Culicoides furens, Spanish purrujas, jejenes) females feed on blood to provide energy for reproduction. They prefer to lay their eggs in waterlogged sand (fresh or brackish) mixed with rotting vegetation just above the high tide line. As JRinPV notes, they don't travel far and are most active in the evening. They can transmit encephalitis. Bites should be treated with antibiotic to prevent infection and local anesthetic. They are unaffected by DEET (the active ingredient in many mosquito repellents) but Avon's skin-so-soft is an excellent repellent. (The US Marines use it by the gallon despite the perfume scent).