Bacteria can cause disease in plants and animals by disrupting homeostasis in the host organism. They can cause disease by invading tissues or producing toxins.
On the Microbiology Online site, read the following pages to understand more on bacteria and diseases they cause:
Antibiotics are used to fight bacterial diseases (remember antibiotics do not work on viruses). However because bacteria can develop a resistance to bacteria, prevention is the best method to fight bacterial diseases. Watch this video on Antibiotic Resistance.