The bacteria changes where it lives in your body, it also can change what proteins are on the surface, which means it requires your body to make new antibodies to fight it with (and also accounts for why sometimes we have to change treatments periodically), and also the bacteria may give off different toxins depending on how they're under attack by treatment, or depending on where they are when they die. The different toxins may affect the body in different ways.
Just my own thoughts on this - not based on scientific research other than the fact about changing surface proteins.
Chronic Lyme Disease, Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD ("Secondary Lupus-Like Syndrome"), Osteoporosis, Anemia, Lymphopenia, intermittant Pancytopenia, chronic malabsorption/malnutrition, etc.; G Tube; Currently TPN-dependent.
Meds: Zofran, Pulmicort, Heparin (to flush PICC line), IV Cipro (for G-tube site infection).