Semen is composed of spermatozoa and seminal plasma. The proportion of spermatozoa in semen is about 5%, and the rest is seminal plasma. In addition to water, fructose, protein and fat, seminal fluid also contains a variety of enzymes and inorganic salts. The physiological function of seminal plasma is to facilitate the survival, activity and transport of sperm. The first part of the ejaculated semen is generally dominated by prostate fluid, in which the sperm density is the highest and the vitality is strong, and the second part is dominated by seminal vesicle secretion, in which the sperm vitality is weak. If the function of the prostate and seminal vesicle undergoes abnormal activities, it may cause male infertility. Normal semen is milky white or yellowish. Semen is a type of body fluid, and many STDs can be transmitted to each other through semen.