It takes one or two weeks for the body to start producing antibodies. While looking for antibodies is a good way to find evidence of past infection, it won't pick up an infection early in the process.
Blood is tested for Ebola by doing a PCR assay, which detects virus RNA. When a person first becomes symptomatic, the PCR test is often negative, but it will be positive a day or so later. Whether this means there is no virus in the blood at that time, or the amount is too low to be detected is an unknown.
Many infections that are diagnosed by PCR of the blood behave the same way--not detectable right away, but are detectable a day or a few days later.
I’m glad that you looked it up.
Here’s the thing: people don’t go to the hospital until they feel sick. But already, the virus has been incubating and multiplying for 6 days, on average. The consequences of the viral load at day 6 are such that the host victim starts feeling sick, and there’s probably a fever.
Depending on the individual circumstances, the host victim may or may not seek medical attention at this point. Within a day or two more, probably.
The doctor/nurse looking at the symptoms may or may not suspect Ebola. Let’s say, it’s the U.S., and the CDC has recommended that blood tests be done, but no more than that.
Two days later the result comes in negative. But the victim is actually positive, and is now gravely ill. A new test is done, and a day or two later is confirmed for Ebola.
Or, the test is done, and comes back two days later, and is positive. The victim, now gravely ill, is quarantined in some manner.
Let’s go back to day 5. Ebola is multiplying, but the victim is still feeling OK—maybe a bit under the weather. He goes across town on public transportation to go bowling.
He’s a carrier of the virus. It’s in his system. That means he’s infectious to some degree other than zero. There are scenarios anyone can concoct that would lead to infection of others at day 5.