Gram negative bacteria (rod-shaped, right?) sap their vitality. I had an expert herp vet team who taught me about it, and this gut infection happened from time to time because I was always introducing outsider iguanas to my home. Yours could have had it from the beginning. The right antibiotic treats it.
Other gut problems are caused by parasites of the type that are treated with Panacur. I think Panacur also kills iguanas' external mites. I'm sure I've read about applying it to the skin to kill external parasites.
Imported iguanas are likely to have one or more different exotic infections that need to be eradicated.
I can't name the various parasites and bad bacteria. After treatment, it's best to give probiotics that are balanced for birds, and are often available from avian vets.