In my current work on living systems, a new understanding of disease is up. Disease is traditionally understood as something negative, to be avoided and gotten rid of.
However, if we see organisms as naturally venturing into the world and encountering challenges from the environment, disease may be seen as the body attempt to deal with challenges that are just too large for regular processing.
Your body wards off bacteria all the time, but if their concentration exceeds a certain threshold, or your threshold has been lowered due to low vitality or suchlike, then the body marshals special resources and puts the body to work, serious work (think inflammation, infection).
Medium-sized challenges the body can handle, pneumonia it may not, and your good doctor steps in with antibiotics to kill off the large challenger which might otherwise have overcome you.