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Q. Why isn’t everyone who arrives from West Africa immediately tested for Ebola? Why don’t we look at their blood under a microscope?

A. Unlike malaria and other diseases caused by worms, eggs and parasites, diseases like Ebola are caused by viruses, which are far too small to be seen under a conventional microscope.

There are blood tests for Ebola, but they are imperfect. And giving them to everyone arriving from West Africa might well backfire, some experts argue, because some new arrivals could pass the strictest test and then still fall ill days later. Because they had tested negative once, they could easily assume that symptoms like fever, nausea and diarrhea were just a flu. They could then potentially spread Ebola until they became so ill that they were forced to go to an emergency room. Because early treatment is important, it could also endanger their lives.

The difficulty is that Ebola has a relatively long incubation period. Although most people show symptoms within four to seven days after infection, some do not show them for up to 21 days. Dr. Craig Spencer, the physician who arrived in New York on Oct. 17, might have been able to pass an Ebola test that day. He did not detect a fever — a classic, although imperfect, warning sign — until Thursday. Kaci Hickox, the nurse who was forced into quarantine at Newark airport on Friday, despite having had a negative Ebola test, might still develop Ebola — or she might never.

There are rapid, cheap tests for Ebola, based on detecting antibodies to the virus, but they work only when a victim has already been ill for several days. Antibodies are produced by the body’s counterattack on the virus, and are therefore not detectable until that counterattack is well underway. By that time, the victim might have infected others.

A test called an RT-PCR, for reverse transcriptase polymerase chain reaction, detects bits of the RNA of the virus itself, so it can be used much earlier in the infection. In some cases, a very sensitive PCR test may detect the virus two days before fever and other symptoms appear. But there is no way to know which people will develop enough virus in their blood to detect soon after they are infected, and which will take many more days.

To be sure, people like Ms. Hickox would have to be tested every day or perhaps every other day until about 19 days after their last possible contact with an Ebola carrier.

That could be cumbersome and expensive. The tests cost $60 to $200 each and typically take about six hours, according to Dr. Siddhartha Mukherjee, an oncologist at Columbia University Medical School and author of “The Emperor of All Maladies,” who favors wider use.

On Saturday, the Food and Drug Administration gave emergency approval to a new PCR test that takes only an hour. But the machines that run it, and the disposable cartridges that blood samples are placed in, are not yet widely distributed.

Also, PCR tests are less than 100 percent accurate, so each test should be run twice — ideally by different laboratories.

Until recently, only the Centers for Disease Control and Prevention and a few other medical school laboratories could do Ebola tests. Now every state health department laboratory can, and more hospitals are getting them. But the tests are not something to be done on tabletops in airports by laypeople. PCR tests are notorious for cross-contamination errors, and the blood of actual Ebola victims is extremely dangerous and must be treated as a biohazard.