一个人中箭了 A man is shot with an arrow and he gets injured, so he has to stay in bed to recuperate.
The oracle bone script of 疾 vividly expresses how a person is injured by an arrow. This is how the character was created.
『說文解字』
病也。从疒矢聲。，古文疾。籀文疾。秦悉切
段玉裁《说文解字注》:“矢能伤人,矢之去甚速,故从矢会意。”本义:受兵伤。泛指疾病