Many years ago, I got into chiggars so bad that I got flu-like symptoms. I had over 300 bites from the waist down.
Nothing seemed to work - fingernail polish, ChiggarX. I had a fever, couldn't eat, couldn't sleep, nausea.
Then, my friend's mom recommended a 10% bleach solution bath for 15 minutes for each day I itched. The first time - 90% of the itching was gone. By day 2 I wasn't sick anymore. By the 3rd treatment - no itching at all. I will never forget that.
When we were kids, after playing in the weeds, we'd go swimming in a pool and never suffered the chiggars.
I also learned to sprinkle 5% Sevin Dust in my most travelled paths. You can also put it on your legs, etc. Now, I spray with Cutter or Repel (not Off) from the knees down.
If I forget, and get a couple of bites, after a warm shower, I soak the bite in 70% alcohol until the sting stops. Then I use hydrocortizone and first aid cream until I'm healed.
And yes - chiggars are microscopic.