Testing negative for a particular disease means that your genetic makeup is unlikely to cause you to contract the condition.  However, it does not mean you will not contract the disease.  This is because exposure to other causes of the disease (environment factors) may also result in the onset of the disease.  Very simply, a negative result implies that one of the main causes in many diseases—the genetic factor, is unlikely to cause the onset of the disease for you.  As such, there is one reduced risk factor.