It very well may be a bug that is resistant to everything you've tried
so far. I mean..... resistant to everything!
Possibly some form of really strong antibiotic will help,
but bugs become resistant to that as well.
We've battled some strange bugs with my quadriplegic son.
He is susceptible to many more things than I knew existed.
We also are dealing with MRSA, & that is a real quandary.