There are some STDs that can have severe consequences and endanger one's life especially in women, if left untreated. Some STDs can lead to pelvic inflammatory disease that can cause infertility while there are others that are considered fatal.

According to the Centers for Disease Control and Prevention there are over 15 million cases of sexually transmitted disease that are reported yearly in the U.S.

These STDs by far are fatal if left untreated:

1. Huma Papilloma Virus is one of the main causes of cervical cancer and has been linked with other types of cancers of the reproductive system especially in females.

2. HIV/AIDS is the one STD that one should be worried about since the infection has a prolonged incubation period and sometimes detected late when it has been transmitted to other person already.

3. Syphilis can easily be treated but this is fatal if ignored since it can progress and affect the nervous system and brain and can lead to dementia and even death.

Of all the STDs, the human immunodeficiency virus (HIV) is the most deadly. In fact, it kills 1 American every six minutes and it has been said to be the sixth leading cause of death among people between the ages of 15 to 24. The major reason why HIV becomes so deadly is that it weakens one’s immune system which makes him more susceptible to diseases like tuberculosis and pneumonia.
Aside from HIV, syphilis and hepatitis (many more) are also known to be very deadly esp. if left untreated. However for HIV, there have been antiretroviral drugs that are said to slow or halt the progression of HIV-related diseases. Many doctors who believe in alternative medicines also would agree that HIV is curable and that I heard too that virgin coconut oil is being used by some AIDS patients and that it has been reported that it reduces viral load in AIDS patients. I also have read that power food supplements (complete Glutathione Peroxidase – Selenium, Aminoacids) can also overcome AIDS.
In my own view, whether the STD is thought to be minor, it should not be taken for granted in order to prevent complications and further damages. And because I have strong faith in alternative medicines and food/nutritional supplements, I guess there's no harm in trying them esp. if they are aimed at strengthening your immune system which is the most important key to fight any disease.

AIDS/HIV can kill you.

Syphilis if not treated can kill you.

Gonorrhea and Chlamydia can cause you to become sterile and can lead to pelvic inflammatory disease which can kill you. http://www.cdc.gov/std/pid/stdfact-pid.htm

Certain types of HPV can cause cervical cancer, which can lead up to death.