A sexually transmitted infection (STI) is passed from one person to another during sexual contact. Chlamydia is an STI caused by bacteria. Many people with chlamydia have no symptoms, so they can pass it on without knowing. If it is not treated it can cause infertility. Because it is a bacterial infection, chlamydia can be treated with antibiotics.
HIV is an STI caused by a virus. It is spread by sexual contact, by sharing infected needles, and from an infected mother to her baby. HIV infects and destroys white blood cells, which weakens the immune system. Without treatment it can lead to the onset of AIDS. Antibiotics do not work on viruses, but antiretroviral drugs can control HIV.
The spread of both infections can be reduced. Using a condom during sex acts as a barrier to the pathogen. Screening lets people find out if they are infected and get treatment. Chlamydia can be prevented from spreading by treating sexual partners, and HIV can be prevented from spreading by never sharing needles.