Is there collaboration between cyber criminals and banks? In disease evolution, one often finds that bacteria and viruses evolve to co-exist with their hosts and become less malevolent. Making an analogy between cybercrime and disease, I am wondering if there can be a symbiotic relationship between banks and cyber criminals?

Here is a more specific example of something I'm wondering about. A cyber criminal with access to a bank's books adds a bunch of car loans in various states of repayment. He then transfers the outstanding amounts of all these loans to himself. Then, at the end of every month, he doctors the bank's books to look like the loan payments have been made. If the bank were to detect such activity, it might be very reluctant to do anything. Doing anything, the result would be default on a large number of authentic looking loans. Doing nothing, the bank would continue to receive for all intents and purposes interest payments on its loans.

To some extent, I have confusion about what is real about any of this money that is changing hands. Can't a cyber criminal increase/decrease a bank's holdings just by changing a number in a computerized ledger book?
The vast majority of cyber crime is fraud. The criminals don't hack the bank directly; rather they hack the customers of the bank and commit fraudulent transactions.

I strongly doubt there is active collaboration. Some criminals may have insider contacts, but that's different to a symbiotic relationship.

There may be an effect somewhat like you mention. Increasing security is expensive for banks, so it is only worth doing if they are experiencing a certain level of losses. In theory it would make sense for criminals to restrict their level of activity, to avoid banks implementing additional security, which would make their job harder. In practice I don't think this happens, because their are multiple cyber gangs and I strongly doubt there is a gentleman's code between then.

It is theoretically possible for someone to hack the bank itself, and directly modify balances - effectively creating money out of thin air. In fact, it is thought that some major nation states (mostly US and China) have penetrated many banks around the word. However, if they started to extract money, this is likely to be noticed, due to internal checks and balances. It is more valuable for these state-sponsored actors to silently remain present, capturing information. And one day, perhaps they will strike, as part of a larger war strategy.