What is the meaning of 病? it is composed of 疒, and 内. So it is related to sickness, and with 内, it should be sickness happening inside (the body). is believed as an indicator to emphasise 内.
病 is to mean sickness happening inside the body, or just sickness.
We can examine its evolution history to double confirm our guessing: (image get from http://www.vividict.com)
|Learn one Chinese Character a day - 病|