The surprising reason some diseases spread so effectively through semen

This week, the Centers for Disease Control and Prevention warned once again that Zika may be able to spread through sexual intercourse. Among other things, the virus seems to be able to persist in semen for weeks after a person is otherwise cleared of an infection.

This isn't the first time researchers have cautioned that viruses can survive for a surprisingly long time in this particular bodily fluid. During the Ebola outbreak, researchers discovered that the virus could persist in semen for months after a patient had recovered. And then there are viruses like HIV that spread very effectively through semen but not through other bodily fluids, like tears or urine.

It raises a question: What's so special about semen, anyway? Why is it such an effective carrier of disease?

Testicles are very friendly hosts for some viruses

Many scientists think this has to do with the fact that the immune system is actually weaker in the testes, allowing viruses to thrive here. This makes sense from an evolutionary perspective. The testes are so important to our survival that they need an extra layer of protection from attacks by the immune system. (A few other crucial body parts are also "immune privileged.")

The "blood-testis barrier" actually blocks blood vessels from the tubes where the sperm are made, so that immune cells can't move from the blood into the sperm. Sarah Fecht does a great job of explaining why this system evolved in Popular Science: "White blood cells see the little spermatozoa as invaders — probably because males don’t start producing semen until after the body’s immune system has already been established."

The dearth of immune cells means that sperm can be especially conducive to holding on to viruses for longer periods of time.

Ebola virus in the eye 14 weeks after onset of the disease. (New England Journal of Medicine)

We saw this system in action during Ebola. There were cases where the virus was found in semen for at least nine months after the infection cleared from other bodily fluids. (They also found Ebola in survivors' eyeballs, another immune-privileged site.)

Scientists are now seeing similar trends with Zika. In the UK, researchers found the virus in a man's semen 62 days after he got sick. Though cases of Zika sexual transmission are now popping up, researchers still aren't clear how common it is compared with transmission from mosquitoes, and when people are most at risk.

Not all viruses behave the same way

Still, Isaac Bogoch, an infectious diseases doctor, cautioned about extrapolating from how Ebola and Zika persist in semen to other pathogens. "It might be true that some viruses persist in the sperm for longer than others — but that's not true for all viruses," he said. "Different viruses behave differently," he added, admitting that researchers still have a lot to learn about how foreign invaders attack various parts of the body and bodily fluids.

Another complicating factor: Just because a virus is found in a certain bodily fluid doesn't mean a person can pass it along. That depends on the "viral load" of the person who is infected — or the number of copies of virus per milliliters of bodily fluid.

In other words, researchers could detect a virus in semen or blood, but that doesn't mean the infected person can pass it along if the virus is only present in very small quantities.