This type of treatment is sometimes used for early or low stage bladder cancer that has high risk for returning (recurrence) or becoming more advanced disease (progression). A weakened bacterium called BCG is placed directly into the bladder. Immune system cells are attracted and activated by BCG, which in turn affects the bladder cancer cells. Treatments are given weekly for six weeks, and may be repeated or used as preventative therapy (maintenance) using three weekly treatments every six months for a few years. Sometimes, a reduced dose of BCG is used with another immune-active agent called interferon, which causes fewer side effects, but maintains effectiveness against tumors that may not have responded to BCG alone.