Urethritis

The urethra is the tube that carries urine out of the body, and in men it carries semen too. When that tube gets inflamed, it's called urethritis. Most often the cause is a sexually transmitted infection, one that spreads through sexual contact.

The inside of the urethra is lined with a layer of cells called the epithelium, a bit like the inner lining of a garden hose. That lining is where the bacteria do their work, and which bacterium is involved decides the name. Gonococcal urethritis is caused by gonococcus, the bacterium Neisseria gonorrhoeae. Nongonococcal urethritis is mainly caused by chlamydia, the bacterium Chlamydia trachomatis. In men, gonococcus makes the cells of that lining release pro-inflammatory cytokines, chemical signals that set inflammation in motion, like an alarm going off inside the wall of the hose. Chlamydia works differently: it gets inside those same cells, multiplies there, and ends up killing them.

The most common symptom is urethral discharge, fluid that comes out of the urethra. There can also be dysuria, which means pain when you pee, and itching. But urethritis is often asymptomatic, meaning it causes no symptoms at all, and that's especially true in women.