Some very useful information can be found on the American Cats Society page: https://journals.sagepub.com/doi/pdf/10.1177/1098612X13477537
This is a common disease of young and middle-aged cats which causes inflammation in the bladder. This results in typical ‘cystitis’ symptoms- frequent squatting and straining to urinate and sometimes blood in the urine. Bouts of cystitis tend to last for several days and tend to recur quite often. In male cats the disease can be dangerous; in males the urethra, which empties the bladder and runs through the penis is only about the width of a sewing needle; it can easily become totally blocked with inflammatory cells and small crystals, leaving the cat unable to pass urine. In males showing these symptoms it is vital to get these cats to the vet urgently to find out if the bladder has become blocked.