Chlamydia is currently one of the most common sexually transmitted diseases - about 4 million cases of chlamydia occur in the USA each year. However, about half of all men and three-quarters of all women who have chlamydia have no symptoms and don't know that they have the disease. The disease is transmitted by the Chlamydia trachomatis bacterium. It can be serious but it is easily cured if detected in time.
Almost half of all women who get chlamydia and arenīt treated by a doctor will get pelvic inflammatory disease (PID), a generic term for infection of the uterus, fallopian tubes, and/or ovaries. PID can cause scarring inside the reproductive organs, which can later cause serious complications, including chronic pelvic pain, difficulty becoming pregnant, ectopic (tubal) pregnancy, and other dangerous complications of pregnancy. Chlamydia causes 250,000 to 500,000 cases of PID every year in the U.S. 
In women, chlamydia may not cause any symptoms, but symptoms that may occur include: unusual vaginal bleeding or discharge, pain in the abdomen, painful sexual intercourse, fever, painful urination or the urge to urinate more frequently than usual.
In men, chlamydia may not cause any symptoms, but symptoms that may occur include: a painful or burning sensation when urinating, an unusual discharge from the penis, swollen or tender testicles, or fever.
Chlamydia in men can spread to the testicles, causing epididymitis, which can cause sterility. Chlamydia causes more than 250,000 cases of epididymitis in the USA each year.
Chlamydia may also cause Reiter's Syndrome, especially in young men. About 15,000 men get Reiter's Syndrome from chlamydia each year in the USA, and about 5,000 are permanently affected by it.
As many as half of all infants born to mothers with chlamydia will be born with the disease. Chlamydia can affect infants by causing spontaneous abortion (miscarriage), premature birth, blindness, and pneumonia.
Fortunately, chlamydia can be effectively cured with antibiotics once it is detected.
Because chlamydia is so common and because it often doesn't produce symptoms, it is especially important to take precautions against sexually transmitted disease by practicing safer sex.