Via his blog epidemic, Dr. Andrew Rambaut posts: Case Fatality Rate for ebolavirus. This is a really good analysis, and you should read the whole thing. Excerpt:
A case fatality rate (CFR) or case fatality risk is a property of an infectious disease in a particular population which states the risk of fatality due to the disease per case.
The first thing to note is that it can't be reliably be calculated for an ongoing epidemic by dividing the reported number of deaths due to a disease by the reported number of cases.
There are a number of reasons why this will be a poor estimate: Firstly, this doesn't take into account infections that have yet to run their course (ending in recovery or death). If many new cases are being reported, then this will under estimate the CFR. A more reliable estimate can be made if the number of recoveries is also being reported.
Secondly, the estimate will be poor if there is a bias in reporting or diagnosis towards severe cases of the disease. This is the case with MERS-CoV in the Middle East where there are many instances of mild or asymptomatic cases (discovered through contact tracing) but most primary cases are only tested if the patients are hospitalized. This will overestimate the CFR.
With ebolavirus, bias may occur if patients are being looked after at home and only being hospitalized or recorded if the disease becomes very severe or if the patient dies.