It is the larvae of the harvest mite (also known as a chigger)
The tiny red bug will infest humans (and other animals) usually while out in tall grass or other such undergrowth.
Red bugs or "chiggers" as they are commonly referred to can cause itching 24-48 hours after contact. Chiggers are found in tall grass and overgrown vegetation. Keep your lawn mowed and landscaping trimmed and maintained. Remove large piles of organic debris and keep mulch less than 2 inches thick.