Cancer is second onil to heart disease aas a cause of death.

首先 你可能打错了2个单词，句子本身应该是：
Cancer is second ill to heart disease as a cause of death.
或者是
Cancer is second only to heart disease as a cause of death.

意思是：
相对心脏病而言，癌症只是导致死亡的第2大疾病。