Sexually transmitted infections (STIs) are infections that are primarily spread through sexual contact. They can be caused by bacteria, viruses, or parasites and may affect anyone who is sexually active. Common STIs include chlamydia, gonorrhea, syphilis, and HIV. Many STIs can be asymptomatic, meaning individuals may not show symptoms but can still transmit the infection to others. Prevention of STIs involves practicing safe sex, such as using condoms and having regular health check-ups. Early detection and treatment are crucial, as untreated STIs can lead to serious health complications, including infertility and increased risk of HIV transmission.