Coins on the ground are best left there. Some people who are ill, and not very nice, may curse a coin by spitting on it and telling the coin to take the illness from them to give to the next person that holds the coin. Usually, the disease is cast onto some sort of kopeck, so it is easy for most people to resist picking them up. After all, it takes about 6,000 kopecks to make a dollar though 300 kopecks can buy a blank booklet of 12 pages when the booklet is on sale. This may allow the person doing the cursing to feel better about it by believing that only a greedy person, and therefore, someone who should be punished, would pick up such a small denomination of money.
Check out The Treasure of Nikolai Nikolaevich: Adventures on the Amur.