To prevent too much hysteria.
Check it out:

News broke that an unidentified nurse that treated now-deceased Ebola patient Thomas Eric Duncan had tested positive for the disease. Her illness was identified via a self-monitoring program and was isolated immediately after she reported a low-grade fever.

CDC Director Thomas Friedan said he was “deeply concerned” about this new infection, and said that there must have been a breach in safety protocol for ebola to have spread. It’s possible that the nurse was infected by improperly removing safety equipment.