Some people may not immediately know if they have a bladder or kidney infection because the symptoms may seem similar. One difference is where the bacteria has formed. If the bacteria formed in the kidney, then it is a kidney infection.
If the bacteria formed in the bladder, then this is a bladder infection. A lot of the kidney infections that should be treated usually come from untreated urinary tract diseases which is why people should always try to avoid getting UTI.
There are also some bladder infections that may be left untreated so the bacteria will travel to the kidney. This will make both the bladder and kidney infected by the untreated bacteria.