The bacterium Chlamydia trachomatis is, alongside gonococci, the most common cause of sexually transmitted diseases. Chlamydia trachomatis can be detected in 5% - 10% of young, sexually active women, but men can also be infected. Coinfections with gonococci occur. The infection can remain asymptomatic for a long time. Symptoms primarily appear on the uterine lining, from where the infection then spreads. In men, an infection usually manifests first as urethritis.

The spirochete Treponema pallidum is the causative agent of syphilis, a sexually transmitted infection of global importance. For several years, an increase in the incidence rate has also been observed in Western countries, with men being particularly affected. Syphilis typically progresses in three stages: a so-called primary chancre (usually a painless ulcer at the point of entry) forms a few days to weeks after infection. In the secondary stage, the disease manifests itself through general symptoms and skin manifestations, and in the tertiary stage (years after the initial infection), damage to the brain and blood vessels can occur.

The Human Immunodeficiency Virus (HIV) belongs to the group of retroviruses and is transmitted through infectious bodily fluids (e.g., semen, vaginal secretions), particularly during unprotected sexual contact. Another common route of infection is the shared use of injection needles, for example, during intravenous drug use. Initial symptoms such as fever, swollen lymph nodes, exanthema, or difficulty swallowing usually last for 1-2 weeks. This is often followed by many symptom-free years until the onset of AIDS (Acquired Immune Deficiency Syndrome). HIV is prevalent worldwide, especially in sub-Saharan African countries, but even in Germany, approximately 2,000 people are newly infected with HIV each year (as of 2020).

The gram-negative bacterium Neisseria gonorrhoeae (gonococcus) is the pathogen responsible for gonorrhea, also known as "the clap." Infection is only possible through sexual contact. The pathogen is widespread globally. Since there is no mandatory reporting in Germany, little epidemiological data is available. The infection can be accompanied by a chlamydia infection. In women, an infection manifests as cervicitis with purulent-mucous discharge. The infection can ascend further and even trigger sepsis. In men, it leads to purulent urethritis, which can also affect the prostate and epididymis.

The single-celled parasite Trichomonas vaginalis is the cause of the infectious disease trichomoniasis. The parasite is found worldwide and is primarily transmitted sexually. In Germany, there is no obligation to report cases. An infection often remains asymptomatic for a long time. If symptoms do occur, they manifest in women as "Trichomonas colpitis" (inflammation of the vaginal lining) with frothy, white-yellow discharge. In men, however, inflammation of the urethra and glans penis usually occurs.

The bacterium Mycoplasma genitalium belongs to the group of mycoplasmas, which are ubiquitous. This group differs primarily in the absence of a cell wall, which means that conventional antibiotics cannot be used. In addition, resistance to common therapeutics is increasing. The pathogen can be sexually transmitted and cause urethral infection. An infection can also be found in patients with other urogenital infections, although it is not clear whether Mycoplasma genitalium is the causative agent.

There are over 200 different types of human papillomavirus. The viruses are divided into low- and high-risk HPV types based on their likelihood of causing cancer. In the case of a long-standing infection with a high-risk HPV type, a malignant tumor can develop (e.g., cervical cancer, cancer of the vagina, penis, or anus). An infection initially shows no symptoms. HPV occurs worldwide and is sexually transmitted. Low-risk HPV types are the cause of genital warts.