There are different types of sexually-transmitted diseases that are prevalent these days. Some are caused by viruses, others by bacteria, and the rest by other types of microorganisms.
Signs and symptoms of STDs

If STDs get inside your body, your health becomes compromised. You become at risk of suffering from various signs and symptoms that may or may not cause serious complications in the long run.

For example, STD symptoms such as sores, rashes, headaches, and fever may go away after some days or weeks, even without you taking some medication, but there are others such as damage to the liver, scars, and lesions that may difficult to fix or are totally untreatable. So, it is advised to always practice safe sex and test for STDs routinely.
Getting treatment early

STDs such as HIV and Hepatitis C are caused by viruses, and they require antiretroviral treatment. In HIV's case, since it attacks the immune system, the treatment has to be administered as early as possible so that the immune system is not endangered. In the case of Hepatitis C, treatment has to be administered early on too so that the liver is not put at risk of suffering from cirrhosis, cancer, and other severe medical complications.

Bacterial STDs such as Chlamydia, syphilis, and gonorrhea are easily treatable with antibiotics, so everyone is advised to undergo testing to be able to cure these infections immediately. If not, they can cause serious damage to various bodily organs.
Test for STDs to avoid all these problems at the earliest chance.