This is the reason why despite having negative testing for Ebola, high risk-individuals should still be very vigilant of their symptoms, so that if they become symptomatic they then avoid ANY risky contact with the public ( self-imposed or forced-quarantine ).
In other words, the PCR test is VERY GOOD, it detects even minute amounts of the virus in the blood stream, IF IT IS INDEED PRESENT in the blood stream!
The problem is that the virus might be present in the body ( meaning that the person is infected) BUT NOT PRESENT in the blood YET, so the test will be negative, even though the person is still infected.
Once the disease advances, the person becomes symptomatic and the virus is released from organs like the spleen and liver, to the blood stream. That is why the patient is not really contagious-thru bodily fluids ( blood, saliva, etc.) until later in the course of the infection.
So if you have been in contact with Ebola patients, meaning that you have a high chance of being infected, even when the PCR test has ben negative, early in the course of the risk period, you need to be very careful to monitor yourself (or be monitored) for symptoms of the disease, so that if the symptoms appear, you can isolate yourself ( or be isolated) until you test negative again.