The term 'Sexually Transmitted Diseases' (abbreviated STDs) refers to a group of illnesses that can be transmitted from one person to another through the sharing of body fluids, including ejaculate ("cum"), vaginal fluids, blood, and other fluids. Apart from sharing similar ways of infecting people, the various diseases compromising the STDs have little in common. They have a variety of different causes (including bacteria and viruses), they produce a variety of symptoms (or absence of symptoms), and they have very different effects on the body when left untreated.
STDs are transmitted when body fluids from an infected person come into intimate contact with another person. As their name implies, the most common route through which this body fluid sharing occurs is sexual activity. All forms of sexual activity may involve sharing of body fluids. Sexual contacts involving any combination of genitals, anus, fingers and/or mouth can place a person at risk.
STDs can also be contracted through sharing of needles used for injecting drugs. It doesn't matter what sort of drugs are being used; an insulin user who shares needles is at risk just as is a heroin user who shares needles. For this reason, people who inject drugs or use needles for other injections (like insulin) should never share needles with other people. If sharing needles is unavoidable, needles should be rinsed with bleach for at least 30 seconds (this will kill HIV, but not Hepatitis C). For more information see this website. Again, it is best to not share needles at all. Each needle should be used only once.
See Your Doctor If You Think You May Have An STD
It is very important that you seek immediate medical attention if you suspect you have an STD. STDs are not benign harmless conditions that go away by themselves. Instead, they frequently can lead to serious health complications for infected persons. Also, an infected person will transmit their disease(s) to other people they are intimate with. For example, untreated HIV can lead to AIDS which is frequently fatal. However, HIV infection can be successfully managed (although not cured) with medicines which greatly prolonging lifespan and quality of life. On the milder side, untreated Gonorrhea can lead to a condition called "Pelvic Inflammatory Disease" which can cause infertility. As Gonorrhea is a treatable disease, such complications can be avoided with proper medical treatment.
Most STDs respond to medical treatments. Some STDs can be cured outright with proper medical attention, while others can be medically managed (but not eliminated). Examples of STDs that can be cured (eliminated) include Gonorrhea, Chlamydia, Syphilis, pubic lice, chancroid and Trichomoniasis. STDs that can be managed (but not cured) include HIV/AIDS, Herpes, and Human Papilloma Virus (the virus that causes genital warts). Even when an STD cannot be cured, it is still worth seeking medical attention. Serious complications of STDs like HIV can be brought under control with proper care, and the risk of infecting others can be similarly managed as well. Seeking medical care for STDs is always the right decision.
As classes of diseases go, STDs have a lot of bad press. STDs have for years been considered a sort of "scarlet letter" or mark of shame suggesting that anyone who might contract them is necessarily promiscuous (has sex with many different partners), and therefore sinful. This is not the case. STDs are biological infections not unlike the common cold (which is also spread through body fluids, in that case via sneezes and unwashed hands). While having multiple sexual partners does raise one's risk of contracting an STD, it is also possible to get an STD in the context of a new monogamous relationship (where one partner already has an STD), or when a partners' extramarital affair brings an STD back into an otherwise monogamous relationship. In any case, while affairs or promiscuous sexuality may be shameful, being infected with a disease should not be. If you feel ashamed by the thought that you might have an STD, please do what you can to overcome that shame sufficiently enough to see your doctor for proper treatment.