It is why flu vaccines can be ineffective by the end of the flu season. It takes about 2 weeks to grow a flu virus in sterile eggs, the most "high tech" solution known to man currently. Every time someone with flu sneezes, they are emitting new multiple strains of the flu virus.
The other problem with Ebola is it can hide in parts of the body where the adaptive immune system does not work, places like the eye and testes. So even though the virus may not show up in test results, it could reappear when someone's immune system is weakened sometime in the future. There are other ways to kill it though. Just for reference, someone I know works at Porton Down in the UK, MOD facility, they told me Ebola is nothing compared to some of the viruses and bacteria they have in their facilities. In other words there are far worse things to be worried about than Ebola.