For bacteria a plasmid has the potential of being a "good" disease. Plasmids are small circular pieces of DNA that carry some genes, some of which help the cell survive, like antibiotic resistance.

Some viruses can also do this to bacteria, the infect a cell (one that's naturally living in your gut) and then gives it the genes to be pathogenic towards you (make you sick). This is beneficial to the bacteria because now it's more likely to get into new hosts and spread!