In an ideal world, health care workers returning from West Africa would get a quick blood test to prove they aren't carrying the Ebola virus. A test like that would likely put to rest some of the anxiety surrounding these doctors, nurses and scientists.

Unfortunately, even the best blood test in the world can't do that.

The test uses a technology called PCR, for polymerase chain reaction. It can detect extraordinarily small traces of genetic material from the Ebola virus.

But the catch is, the test is usually used on blood samples. And in the beginning, that's not where the Ebola virus hides.

"The initial sites of replication actually are not in the blood itself — they're mostly in tissues like spleen or liver," says Thomas Geisbert, a microbiologist at the University of Texas Medical Branch in Galveston.

It's not practical to sample these organs to look for Ebola. But the virus doesn't stay there forever, Geisbert explains. As the infection grows, virus particles are gradually released into the blood, as well.

And as soon as a small amount of virus ends up in the blood, PCR will detect it. It can find one or two virus particles in a drop of blood. That concentration is so low, Geisbert says, that an infected person's body fluids pose a minuscule risk to others at that stage. The problem is, that can change pretty quickly.

"As the disease progresses, and people start to get sick," he says, "in that same small drop of blood [there can be] 100 particles — or a thousand particles."

That's the point when body fluids do pose a risk. It's also the moment when the infected person starts to feel sick.

"You're going to start to detect the virus at about the same time you're going to have clinical signs of disease," Geisbert says.

So — sensitive as the PCR test is — it doesn't reliably give you much advance warning that a person is infected.

On the plus side, this pattern of infection also explains why people infected with Ebola aren't a risk to others until they actually fall ill with symptoms.

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