You've actually got two separate grammar points here. One, as you know is of passive voice, but the other is of verbals, specifically infinitives.
In the sentence "Iodine deficiency is an easy and inexpensive nutrient disorder to prevent," the phrase 'to prevent' is an infinitive. One of the ways we use infinitives is after [to be] [adjective], as in "is easy and inexpensive to prevent," or "is easy to do."*
Furthermore, in that sentence, the primary verb connected with the subject is [to be], which of course is a special verb in many ways, one of which is that it doesn't give the subject agency. A subject with agency (in other words, an active subject) is what gives a sentence active voice. Passive voice is all about using a noun that isn't the agent (isn't active) as the subject.**
When your subject isn't an agent, changing its sentence into a passive one is redundant at best (I question whether it's even possible to do correctly). And when you do change a sentence to passive structure, you do so by modifying the main verb ('is'), not a verbal ('to prevent').
Your original sentence is fine as it stands.
*see https://learnenglish.britishcouncil.org/en/english-grammar/verbs/infinitive or To infinitive used after adjective