Is a stab wound to the back lethal?
Check out the sources of Wikipedia article: the mortality rate is up to 4%.

The main causes of "immediate" death in penetrating trauma are shock (low blood pressure due to external/internal hemorrhage, especially arterial hemorrhage), pneumothorax, penetrating heart injury (with resultant hemopericardium) and other less frequent causes. 
The same causes account for "later" lethal case plus some specific causes like infections, emboly etc. 

The overall mortality rate of stab wounds is far from 100%, and according to different studies is between 1% to 4%, but rarely more. The morbidity depends on depth of wound, place of injury, overall health status etc, thus multifactorial analysis is required to get an overall image of the penetrating wounds of the body. 

Some sources:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2627025/ - 
 (4.3% mortality in stab wounds to the anterior abdomen, back, and flank). 

http://www.ncbi.nlm.nih.gov/pubmed/3798182 - the mortality in back/flank stab wounds was 1.3%.