A lot of people skip running as a form of cardio because they are worried it will ruin their knees later in life. Running with an injury (or inflaming a previous injury) can definitely lead to further injuries (ITB Syndrome, runners knee or even arthritis) but there are ways to keep those knees safe...
Abnormal joint stress can increase your risk for arthritis and can be caused by running on an injured joint, poor biomechanics, or muscle weakness due to inadequate cross-training. 
