So let us examine its evolution history first (image get from http://www.vividict.com) -
|Learn one Chinese Character a day - 疒|
Now you might have already shout out is lying on bed. True. There are mostly two conditions we would like to lying on bed, one reason is we are tired, another is because of sickness.
As learnt in our last lesson that take a rest is already presented by 休, so was used to represent the meaning of lying on bed because of sickness.
Seal Script simplified part to an indicator , and eventually becomes 疒.
疒 is now only used as a Chinese Radical (https://en.wikipedia.org/wiki/Radical_(Chinese_characters)). When you see any Chinese character with 疒, they are all related to sickness.
Since 疒 is now only used as Radical, in today's Calligraphy, we use a Chinese character with 疒 as a replacement: