Pain

All creatures feel pain. Pain sensation originates either in the skin, bone, join and muscle, or in the internal organs. Pain is typically evidenced by decreased use of the painful part including overall decreased activity. Dogs may cry out in pain, especially if the pain is intermittent and sharp. Dogs may not cry out even when in severe pain if the pain is dull, aching, and continuous.

Indicators of the presence of pain include:

attempts to decrease use of the painful part, e.g., limping
reluctance to move neck or back
reluctance to change body position or to assume a particular body position
withdrawal from all unnecessary activity

Touching a painful area my elicit responses including:

moving away from the touch
tightening of muscles in the area
aggressive behavior such as growling or biting

The presence of pain does not provide much information with respect to the disease-causing agent. It is mainly an indicator that further diagnostic investigation is required to identify the underlying cause.

Be aware that a dog exhibiting aggressive behavior as a result of pain will require restraining before being transported to a clinic. You don't want to end up in the hospital as well!