Specific age groups, persons who participate in risky sexual behavior, or those have certain health conditions may require screening. The CDC recommends that sexually active women under the age of 25 and those over 25 at risk should be screened for chlamydia and gonorrhea yearly. Appropriate times for screening are during regular pelvic examinations and preconception evaluations. Nucleic acid amplification tests are the recommended method of diagnosis for gonorrhea and chlamydia. This can be done on either urine in both men and women, vaginal or cervical swabs in women, or urethral swabs in men. Screening can be performed:

- to assess the presence of infection and prevent tubal infertility in women

- during the initial evaluation before infertility treatment

- to identify HIV infection

- for men who have sex with men

- for those who may have been exposed to hepatitis C

- for HCV

The United States Preventive Services Task Force (USPSTF) recommends screening for gonorrhea in women at increased risk of infection, which includes all sexually active women younger than 25 years. Extragenital gonorrhea and chlamydia are highest in men who have sex with men (MSM). Additionally, the USPSTF also recommends routine screening in people who have previously tested positive for gonorrhea or have multiple sexual partners and individuals who use condoms inconsistently, provide sexual favors for money, or have sex while under the influence of alcohol or drugs.

Screening for gonorrhea in women who are (or intend to become) pregnant, and who are found to be at high risk for sexually transmitted diseases, is recommended as part of prenatal care in the United States.

Testing may be for a single infection, or consist of a number of tests for a range of STIs, including tests for syphilis, trichomonas, gonorrhea, chlamydia, herpes, hepatitis and HIV. No procedure tests for all infectious agents.

STI tests may be used for a number of reasons:

- as a diagnostic test to determine the cause of symptoms or illness

- as a screening test to detect asymptomatic or presymptomatic infections

- as a check that prospective sexual partners are free of disease before they engage in sex without safer sex precautions (for example, when starting a long term mutually monogamous sexual relationship, in fluid bonding, or for procreation).

- as a check prior to or during pregnancy, to prevent harm to the baby

- as a check after birth, to check that the baby has not caught an STI from the mother

- to prevent the use of infected donated blood or organs

- as part of the process of contact tracing from a known infected individual

- as part of mass epidemiological surveillance

Early identification and treatment results in less chance to spread disease, and for some conditions may improve the outcomes of treatment. There is often a window period after initial infection during which an STI test will be negative. During this period, the infection may be transmissible. The duration of this period varies depending on the infection and the test. Diagnosis may also be delayed by reluctance of the infected person to seek a medical professional. One report indicated that people turn to the Internet rather than to a medical professional for information on STIs to a higher degree than for other sexual problems.

Use of male condoms or female condoms may help prevent the spread of trichomoniasis, although careful studies have never been done that focus on how to prevent this infection. Infection with Trichomoniasis through water is unlikely because "Trichomonas vaginalis" dies in water after 45–60 minutes, in thermal water after 30 minutes to 3 hours and in d urine after 5–6 hours.

Currently there are no routine standard screening requirements for the general U.S. population receiving family planning or STI testing. The Centers for Disease Control and Prevention (CDC) recommends Trichomoniasis testing for females with vaginal discharge and can be considered for females at higher risk for infection or of HIV-positive serostatus.

The advent of new, highly specific and sensitive trichomoniasis tests present opportunities for new screening protocols for both men and women. Careful planning, discussion, and research are required to determine the cost-efficiency and most beneficial use of these new tests for the diagnosis and treatment of trichomoniasis in the U.S., which can lead to better prevention efforts.

A number of strategies have been found to improve follow-up for STI testing including email and text messaging as reminders of appointments.

Traditionally, gonorrhea was diagnosed with gram stain and culture; however, newer polymerase chain reaction (PCR)-based testing methods are becoming more common. In those failing initial treatment, culture should be done to determine sensitivity to antibiotics. All people testing positive for gonorrhea should be tested for other sexually transmitted diseases such as chlamydia, syphilis, and human immunodeficiency virus.

There are three main ways to test for Trichomoniasis.

- The first is known as saline microscopy. This is the most commonly used method and requires an endocervical, vaginal, or penile swab specimen for examination under a microscope. The presence of one or multiple trichomonads constitutes a positive result. This method is cheap but has a low sensitivity (60-70%) often due to an inadequate sample, resulting in false negatives.

- The second diagnostic method is culture, which has historically been the "gold standard" in infectious disease diagnosis. Trichomonas Vaginalis culture tests are relatively cheap; however, sensitivity is still somewhat low (70-89%).

- The third method includes the nucleic acid amplification tests (NAATs) which are more sensitive. These tests are more costly than microscopy and culture, and are highly sensitive (80-90%).

The Nugent Score is now rarely used by physicians due to the time it takes to read the slides and requires the use of a trained microscopist. A score of 0-10 is generated from combining three other scores. The scores are as follows:

- 0–3 is considered negative for BV

- 4–6 is considered intermediate

- 7+ is considered indicative of BV.

At least 10–20 high power (1000× oil immersion) fields are counted and an average determined.

DNA hybridization testing with Affirm VPIII was compared to the Gram stain using the Nugent criteria. The Affirm VPIII test may be used for the rapid diagnosis of BV in symptomatic women but uses expensive proprietary equipment to read results, and does not detect other pathogens that cause BV, including Prevotella spp, Bacteroides spp, & Mobiluncus spp.

To make a diagnosis of bacterial vaginosis, a swab from inside the vagina should be obtained. These swabs should be tested for:

- A characteristic "fishy" odor on wet mount. This test, called the "whiff test", is performed by adding a small amount of potassium hydroxide to a microscopic slide containing the vaginal discharge. A characteristic fishy odor is considered a positive whiff test and is suggestive of bacterial vaginosis.

- Loss of acidity. To control bacterial growth, the vagina is normally slightly acidic with a pH of 3.8–4.2. A swab of the discharge is put onto litmus paper to check its acidity. A pH greater than 4.5 is considered alkaline and is suggestive of bacterial vaginosis.

- The presence of "clue cells" on wet mount. Similar to the whiff test, the test for clue cells is performed by placing a drop of sodium chloride solution on a slide containing vaginal discharge. If present, clue cells can be visualized under a microscope. They are so-named because they give a clue to the reason behind the discharge. These are epithelial cells that are coated with bacteria.

Two positive results in addition to the discharge itself are enough to diagnose BV. If there is no discharge, then all three criteria are needed.

Differential diagnosis for bacterial vaginosis includes the following:

- Normal vaginal discharge.

- Candidiasis (thrush, or a yeast infection).

- Trichomoniasis, an infection caused by "Trichomonas vaginalis".

- Aerobic vaginitis

The Center For Disease Control (CDC) defines STIs as "a variety of clinical syndromes and infections caused by pathogens that can be acquired and transmitted through sexual activity." But the CDC does not specifically identify BV as sexually transmitted infection.

Mucopurulent cervicitis (MPC) is characterized by a purulent or mucopurulent endocervical exudate visible in the endocervical canal or in an endocervical swab specimen. Some specialists also diagnose MPC on the basis of easily induced cervical bleeding. Although some specialists consider an increased number of polymorphonuclear white blood cells on endocervical Gram stain as being useful in the diagnosis of MPC, this criterion has not been standardized, has a low positive-predictive value (PPV), and is not available in some settings. MPC often is without symptoms, but some women have an abnormal vaginal discharge and vaginal bleeding (e.g., after sexual intercourse). MPC can be caused by "Chlamydia trachomatis" or "Neisseria gonorrhoeae"; however, in most cases neither organism can be isolated. MPC can persist despite repeated courses of antimicrobial therapy. Because relapse or reinfection with "C. trachomatis" or "N. gonorrhoeae" usually does not occur in persons with persistent cases of MPC, other non-microbiologic determinants (e.g., inflammation in the zone of ectopy) might be involved.

Patients who have MPC should be tested for "C. trachomatis" and for "N. gonorrhoeae" with the most sensitive and specific test available. However, MPC is not a sensitive predictor of infection with these organisms; most women who have "C. trachomatis" or "N. gonorrhoeae" do not have MPC.

Diagnosis is typically suspected based on a women's symptoms. Diagnosis is made with microscopy (mostly by vaginal wet mount) and culture of the discharge after a careful history and physical examination have been completed. The color, consistency, acidity, and other characteristics of the discharge may be predictive of the causative agent. Determining the agent is especially important because women may have more than one infection, or have symptoms that overlap those of another infection, which dictates different treatment processes to cure the infection. For example, women often self-diagnose for yeast infections but due to the 89% misdiagnosis rate, self-diagnoses of vaginal infections are highly discouraged.

Another type of vaginitis, called desquamative inflammatory vaginitis (DIV) also exists. The cause behind this type is still poorly understood. DIV corresponds to the severe forms of aerobic vaginitis. About 5 to 10% of women are affected by aerobic vaginitis.

The International Statistical Classification of Diseases and Related Health Problems codes for the several causes of vaginitis are:

Prevention of candidiasis, the most common type of vaginitis, includes using loose cotton underwear. The vaginal area should be washed with water. Perfumed soaps, shower gels, and vaginal deodorants should be avoided. Douching is not recommended. The practice upsets the normal balance of yeast in the vagina and does more harm than good.

Prevention of bacterial vaginosis includes healthy diets and behaviors as well as minimizing stress as all these factors can affect the pH balance of the vagina.

Prevention of trichomoniasis revolves around avoiding other people's wet towels and hot tubs, and safe-sex procedures, such as condom use.

Some women consume good bacteria in food with live culture, such as yogurt, sauerkraut and kimchi, or in probiotic supplements either to try to prevent candidiasis, or to reduce the likelihood of developing bacterial vaginitis following antibiotic treatment. There is no firm evidence to suggest that eating live yogurt or taking probiotic supplements will prevent candidiasis.

Studies have suggested a possible clinical role for the use of standardized oral or vaginal probiotics in the treatment of bacterial vaginosis, either in addition to or in place of the typical antibiotic regimens. However, recent articles question their efficacy in preventing recurrence compared with other means, or conclude that there is insufficient evidence for or against recommending probiotics for the treatment of bacterial vaginosis.

Cervicitis can be caused by any of a number of infections, of which the most common are chlamydia and gonorrhea, with chlamydia accounting for approximately 40% of cases. As many half of pregnant women are asymptomatic with a gonorrhea infection of the cervix. "Trichomonas vaginalis" and herpes simplex are less common causes of cervicitis. There is a consistent association of M. genitalium infection and female reproductive tract syndromes. M. genitalium infection is significantly associated with increased risk of cervicitis.

In female patients, urethritis can be caused by pelvic inflammatory disease.

In males, thepenis and testicles may show signs of pain and swelling. The urethra is visually examined by spreading the urinary meatus apart with two gloved fingers, and examining the opening for redness, discharge and other abnormalities. Next, a cotton swab is inserted 1-4 cm into the urethra and rotated once. To prevent contamination, no lubricant is applied to the swab, which can result in pain or discomfort. The swab is then smeared onto a glass slide and examined under a microscope. A commonly used cut-off for the diagnosis of urethritis is 5 or more granulocytes per High Power Field, but this definition has recently been called into doubt. The physician sometimes performs a digital rectal examination to inspect the prostate gland for swelling or infection.

A urinary tract infection may cause similar symptoms.

About 5-8% of the reproductive age female population will have four or more episodes of symptomatic Candida infection per year; this condition is called recurrent vulvovaginal candidiasis (RVVC). Because vaginal and gut colonization with Candida is commonly seen in people with no recurrent symptoms, recurrent symptomatic infections are not simply due to the presence of Candida organisms. There is some support for the theory that RVVC results from an especially intense inflammatory reaction to colonization. Candida antigens can be presented to antigen presenting cells, which may trigger cytokine production and activate lymphocytes and neutrophils that then cause inflammation and edema.

Risk of some causes of urethritis can be lessened by avoiding unprotected sexual activity, chemicals that could irritate the urethra; this could include detergents or lotions as well as spermicides or contraceptives, and irritation caused by manual manipulation of the urethra.

For infrequent recurrences, the simplest and most cost-effective management is self-diagnosis and early initiation of topical therapy. However, women whose condition has previously been diagnosed with candidal vulvovaginitis are not necessarily more likely to be able to diagnose themselves; therefore, any woman whose symptoms persist after using an over the counter preparation, or who has a recurrence of symptoms within 2 months, should be evaluated with office-based testing. Unnecessary or inappropriate use of topical preparations is common and can lead to a delay in the treatment of other causes of vulvovaginitis, which can result in worse outcomes.

When there are more than four recurrent episodes of candidal vulvovaginitis per year, a longer initial treatment course is recommended, such as orally administered fluconazole followed by a second and third dose 3 and 6 days later, respectively.

Other treatments after more than four episodes per year, may include ten days of either oral or topical treatment followed by fluconazole orally once per week for 6 months. About 10-15% of recurrent candidal vulvovaginitis cases are due to non-"Candida albicans" species. Non-"albicans" species tend to have higher levels of resistance to fluconazole. Therefore, recurrence or persistence of symptoms while on treatment indicates speciation and antifungal resistance tests to tailor antifungal treatment.

Sexually transmitted disease that affect the vagina include:

- Herpes genitalis. The herpes simplex virus (HSV) can infect the vulva, vagina, and cervix, and this may result in small, painful, recurring blisters and ulcers. It is also common for there to be an absence of any noticeable symptoms.

- Gonorrhea

- Chlamydia

- Trichomoniasis

- Human papillomavirus (HPV), which may cause genital warts.

HIV/AIDS can be contracted through the vagina during vaginal intercourse, but it is not associated with any local vaginal or vulval disease.

Because of STIs, health authorities and other health outlets recommend safe sex practices when engaging in sexual activity.

A vaginal yeast infection results from overgrowth of candida albicans, or yeast, in the vagina. This is a relatively common infection, with over 75% of women having experienced at least one yeast infection at some point in their life. Risk factors for yeast infections include recent antibiotic use, diabetes, immunosuppression, increased estrogen levels, and use of certain contraceptive devices including intrauterine devices, diaphragms, or sponges. It is not a sexually transmitted infection. Candida vaginal infections are common; an estimated 75% of women will have at least one yeast infection in their lifetime. Vaginal discharge is not always present in yeast infections, but when occurring it is typically odorless, thick, white, and clumpy. Vaginal itching is the most common symptom of candida vulvovaginitis. Women may also experience burning, soreness, irritation, pain during urination, or pain during sex. The diagnosis of Candida vulvovaginitis is made by looking at a sample taken from the vagina under the microscope that shows hyphae (yeast), or from a culture. It is important to note that the symptoms described above may be present in other vaginal infections, so microscopic diagnosis or culture is needed to confirm the diagnosis. Treatment is with intra-vaginal or oral anti-fungal medications.

HIV/AIDS is diagnosed via laboratory testing and then staged based on the presence of certain signs or symptoms. HIV screening is recommended by the United States Preventive Services Task Force for all people 15 years to 65 years of age including all pregnant women. Additionally, testing is recommended for those at high risk, which includes anyone diagnosed with a sexually transmitted illness. In many areas of the world, a third of HIV carriers only discover they are infected at an advanced stage of the disease when AIDS or severe immunodeficiency has become apparent.

Most people infected with HIV develop specific antibodies (i.e. seroconvert) within three to twelve weeks of the initial infection. Diagnosis of primary HIV before seroconversion is done by measuring HIV-RNA or p24 antigen. Positive results obtained by antibody or PCR testing are confirmed either by a different antibody or by PCR.

Antibody tests in children younger than 18 months are typically inaccurate due to the continued presence of maternal antibodies. Thus HIV infection can only be diagnosed by PCR testing for HIV RNA or DNA, or via testing for the p24 antigen. Much of the world lacks access to reliable PCR testing and many places simply wait until either symptoms develop or the child is old enough for accurate antibody testing. In sub-Saharan Africa as of 2007–2009 between 30 and 70% of the population were aware of their HIV status. In 2009, between 3.6 and 42% of men and women in Sub-Saharan countries were tested which represented a significant increase compared to previous years.

Trichomonas vaginitis is an infection acquired through sex that is associated with vaginal discharge. It can be transmitted by way of the penis to the vagina, the vagina to the penis, or from vagina to vagina. The discharge in Trichomonas is typically yellowish-green in color. It sometimes is frothy and can have a foul smell. Other symptoms may include vaginal burning or itching, pain with urination, or pain with sexual intercourse. Trichomonas is diagnosed by looking at a sample of discharge under the microscope showing trichomonads moving on the slide. However, in women with trichomonas the organism is typically detected in only 60-70% of cases. Other testing, including a culture of the discharge or a PCR assay, are more likely to detect the organism. Treatment is with a one time dose of oral antibiotics, most commonly metronidazole or tindazole.

A vaginal disease is a pathological condition that affects part or all of the vagina. Some can be prevented by vulvovaginal health maintenance.

Penile discharge is fluid that comes from the urethra at the end of the penis. This may occur with infections due to gonorrhea, chlamydia, or trichomoniasis. In gonorrhea the discharge may by white, yellow, or green.

It may also result from inflammation or congestion of the vaginal mucosa. In cases where it is yellowish or gives off an odor, a doctor should be consulted since it could be a sign of several disease processes, including an organic bacterial infection (aerobic vaginitis) or STD.

After delivery, leukorrhea accompanied by backache and foul-smelling lochia (post-partum vaginal discharge, containing blood, mucus, and placental tissue) may suggest the failure of involution (the uterus returning to pre-pregnancy size) due to infection. A number of investigation such as wet smear, Gram stain, culture, pap smear and biopsy are suggested to diagnose the condition.

Leukorrhea is also caused by trichomonads, a group of parasitic protozoan, specifically "Trichomonas vaginalis". Common symptoms of this disease are burning sensation, itching and discharge of frothy substance, thick, white or yellow mucous.