What can cause a bladder infection usually?

Bacteria. : Urinary tract infections occur when pathogenic bacteria proliferate in the urine, causing inflammation in the urethra, bladder and sometimes the kidney. E. Coli is the most common cause, which is usually found in the colon but can contaminate the urine since the urethra and anus are in such close proximity. Infections are cleared with antibiotics, and should be confirmed by urine culture.