Illness is not only caused by viruses and bacteria, but also by the negativities we harbor in the mind. The Vedic philosophy lays great emphasis on the ramification of thoughts. We all know people who had everything favorable in the world, but lived miserable lives because their own mind tormented them incessantly through depression, hatred, tension, anxiety, and stress. The external demons may injure us for some time, but the demons sitting within our own mind have the ability to make us live in constant wretchedness.
These internal enemies are even more pernicious than the outer ones.
The Vedic scriptures say the biggest enemies-lust, anger, greed, envy, illusion, etc.-reside in our own mind. We dissipate a large portion of our thought power and energy in combating people whom we perceive as enemies and potentially harmful to us.