Question

What sexually transmitted disease is so common that 50 percent of sexually active adults contract it at some point in their lifetime?

Answer 1

That would be Herpes Simplex: 40-60% of the population in the "developed" world, 70-80% in the rest.

Explanation:

Herpes Simplex is a member of the Herpesviridae, a large class of DNA viruses: Over 130 have been identified, though only about 9 have been shown to infect humans.

Some of them are quite nasty, but the Simplex-variety is not life-threating though it can make life uncomfortable.

There are two types: One infects the area around the mouth type, known as Herpes Simplex Virus 1, or HSV-1 , the other ( HSV-2 ) infects the genital area(s).

The resulting infections are called Genital Herpes ( HSV-2) , and Orolabial Herpes , orolabial indicating the lips of the mouth.

Viruses can have two lifecycles: the Lytic and the Lysogenic pathways.

Although these are predominantly observed in bacterial viruses (called phages ), members of the Herpes family of viruses use them as well...

So what's the difference?

Lysogenic pathway:
In the Lysogenic cycle the viral DNA is incorporated into the (nuclear) DNA of the Host. It stays there in a dormant state: No transcription of the viral DNA occurs, and consequently its genes are not expressed. When the cell divides however and the hosts DNA is replicated, the viral genes are automatically copied as well and inherited by the daughter cells. As mentioned, the virus is in a dormant state and no symptoms will occur.

Lytic pathway:
Expression of the viral genes can be triggered: in the case of Herpes Simplex these triggers are most often stress, immune disorders and UV-light, the latter especially in the case of Orolabial Herpes. It results in the viral genes becoming active, resulting in the production of (thousands of) new virus particles that will eventually lyse the host cell (hence " Lytic " pathway)

In the case of Labial Herpes triggering will result in an appearance of blisters, that after cracking will form little ulcers. ("Cold Sores"). Fever and swollen lymphnodes will often occur as well..

In the case of Genital Herpes the symptoms will be more subtle: sometimes a minor sensation of pain, and/or little blisters resulting in ulcers like in Labial Herpes.

After a Lytic "outburst" the virus usually reverts to its dormant lysogenic state: the infection will remain for the rest of your life, as the viral genome is hidden within the host's genome.

Herpes Simplex infection CAN result in more serious afflictions, like lesions on the fingers, infection of the eye, or even HSE ( Encephalitis : infection of the CNS, usually the brain ). Especially the latter one is very serious: without treatment about 70% result in rapid death, the survivors usually have severe neurological damage....

Thankfully these more serious cases don't occur often: the estimates for HSE range from 2 to 60 per million....