To put the words into the correct order to form a sentence, we need to understand the meaning of the sentence.
The given words are: "Isn't his immune system strong because he often isn't sick."
Step 1: Identify the intended meaning.
The sentence seems to question the strength of 'his immune system' by suggesting that 'he often isn't sick.'
Step 2: Rearrange the words.
By thinking about how questions are typically structured in English, the correct order of words should reflect a question form or a strengthening statement.
A possible sentence could be: "Isn't his immune system strong because he isn't often sick?"
In this order, the sentence becomes a question asking if the reason he isn't often sick is due to his strong immune system.