The difficulty is locked on hard unless you pay money, point blank. Your doctors have levels, and every few rounds the infection level will increase. I can't keep up with it, not on endless and not on regular rounds. It's insane. You have to spend real money to keep up, or play the same level about fifty times to get the amount of coins you need to buy an upgrade that will just scrape you by that level goal. Infection level went to 11 and my doctors are only on 7, so I'm done. Scummy.