"Human sexuality" refers to people's sexual interest in and attraction to others, and the capacity to have erotic or sexual feelings and experiences. Sexuality has biological, emotional, and sociocultural aspects, all of which can influence various sexual disorders and diseases.

Sexual Disorders

The World Health Organization's International Classifications of Diseases defines sexual problems as "the various ways in which an individual is unable to participate in a sexual relationship as he or she would wish." Sexual disorders, according to the 5th edition of the American Psychiatric Association's Diagnostic and Statistical Manual (DSM-5), are disturbances in sexual desire and psycho-physiological changes that characterize the sexual response cycle and cause marked distress and interpersonal difficulty. The DSM-5 includes sex-specific sexual dysfunctions, and subtypes for all disorders include "lifelong versus acquired" and "generalized versus situational." Sexual dysfunctions in the DSM-5 (except those that are substance- or medication-induced) now require a duration of at least 6 months and must meet more exact criteria in order to be diagnosed.

Sexual problems are often categorized in one of four ways: desire disorders, arousal disorders, orgasmic disorders, and sexual pain disorders.

Sexual Desire

Sexual desire disorders, or decreased libido, are characterized by a lack or absence of desire for sexual activity or of sexual fantasies. The condition may have started after a period of normal sexual functioning or the person may always have had low or no sexual desire. The causes vary considerably, but include a possible decrease in the production of normal estrogen in women or testosterone in both men and women. Other causes may include aging, fatigue, hormone imbalance, pregnancy, postpartum depression, medications (such as SSRIs), or psychiatric conditions such as depression and anxiety. Classifying the lack of sexual desire as a "disorder" is considered by some to be controversial because it pathologizes those who are asexual.

Sexual Arousal

Sexual arousal disorders were previously known as frigidity in women and impotence in men, though these have now been replaced with less judgmental terms, such as erectile dysfunction. In the revisions to the DSM-5, sexual desire and arousal disorders in females were combined into female sexual interest/arousal disorder. These conditions can manifest themselves as an aversion to and avoidance of sexual contact with a partner. In males, there may be partial or complete failure to attain or maintain an erection or a lack of sexual excitement and pleasure in sexual activity. There may be medical causes to these disorders, such as decreased blood flow or lack of vaginal lubrication. Chronic disease can also contribute, as can a history of trauma and the nature of the relationship between partners. 

Orgasm Disorders

Orgasm disorders are persistent delays or absence of orgasm following a normal sexual excitement phase. The disorder can have physical, psychological, or pharmacological origins. SSRI antidepressants are a common pharmaceutical culprit, as they can delay orgasm or eliminate it entirely.

Sexual Pain

Sexual pain disorders affect women almost exclusively, raising the question of possible societal influences. In the DSM-5, the conditions of dyspareunia and vaginismus were combined into the new diagnosis of genito-pelvic pain/penetration disorder. Dyspareunia, or painful intercourse, may be caused by insufficient lubrication (vaginal dryness) in females. Poor lubrication may result from insufficient excitement and stimulation; hormonal changes caused by menopause, pregnancy, or breast-feeding; irritation from contraceptive creams and foams; or fear, anxiety, or past sexual trauma. Vaginismus is an involuntary spasm of the muscles of the vaginal wall that interferes with intercourse. It is unclear exactly what causes it, but it is thought that past sexual trauma may play a role. Another female sexual-pain disorder is called vulvodynia or vulvar vestibulitis; in this condition, women experience burning pain during sex, which seems to be related to problems with the skin in the vulvar and vaginal areas. The cause, again, is unclear.

Examining the Causes of Sexual Disorders

There are many factors that may result in a person experiencing a sexual dysfunction. Emotional factors include interpersonal or psychological problems and can result from depression, anxiety, past sexual trauma, sexual fears, or guilt. Ordinary anxiousness can cause erectile dysfunction without psychiatric problems, but clinically diagnosable disorders such as panic disorder commonly cause avoidance of intercourse and premature ejaculation. Pain during intercourse is often correlated with anxiety disorders among females.

Physical factors include the use of drugs, such as alcohol, nicotine, narcotics, stimulants, antihypertensives, antihistamines, and some psychotherapeutic drugs. Other factors include back injuries, enlarged prostate gland, problems with blood supply, and nerve damage (as in spinal cord injuries). Diseases such as diabetic neuropathy, multiple sclerosis, tumors, and tertiary syphilis may also have an impact, as can failure of various organ systems (such as the heart and lungs), endocrine disorders (thyroid, pituitary, or adrenal gland problems), and hormonal deficiencies (low testosterone, other androgens, or estrogen).

Sexually Transmitted Infections

Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) or venereal diseases (VDs), are illnesses that have a significant probability of transmission by means of sexual behavior, including vaginal intercourse, anal sex, and oral sex. Some STIs can also be contracted by sharing intravenous drug needles with an infected person, as well as through childbirth or breastfeeding. Common STIs include:

• chlamydia;
• herpes (HSV-1 and HSV-2);
• human papillomavirus (HPV);
• gonorrhea;
• syphilis;
• trichomoniasis;
• HIV (human immunodeficiency virus) and AIDS (acquired immunodeficiency syndrome).

The most effective way to prevent transmission of STIs is to practice safe sex and avoid direct contact of skin or fluids which can lead to transfer with an infected partner. Proper use of safe-sex supplies (such as male condoms, female condoms, gloves, or dental dams) reduces contact and risk and can be effective in limiting exposure; however, some disease transmission may occur even with these barriers.

Safe sex practices

The use of barriers, such as dental dams for oral sex, can help prevent the spread of STIs.