Ruth 1:1 Now it came to pass in the days when the judges ruled, that there was a famine in the land. And a certain man of Bethlehemjudah went to sojourn in the country of Moab, he, and his wife, and his two sons. 2 And the name of the man was Elimelech, and the name of his wife Naomi, and the name of his two sons Mahlon and Chilion, Ephrathites of Bethlehemjudah. And they came into the country of Moab, and continued there. 3 And Elimelech Naomi’s husband died; and she was left, and her two sons. 4 And they took them wives of the women of Moab; the name of the one was Orpah, and the name of the other Ruth: and they dwelled there about ten years. 5 And Mahlon and Chilion died also both of them; and the woman was left of her two sons and her husband.
In the text we are not told the cause of death of Elimelech and his two sons.
It could have been natural causes or disease.