Should "Todos devem ter informação sobre proteção contra doenças" also be correct? If not, when should "informação" be singular and when should it be plural?
In general, informação is used in plural. But you may also hear "uma informação, nenhuma informação, alguma informação, etc"
Could someone explain why informação is plural here and proteção is singular? Is that just generally how they are used?
Yes. "Informações" is usually plural in Portuguese since it often involves more than one piece of information.
Why is dever in the present instead of the future tense here? Earlier "he must have" was translated as "ele deverá".
why is it wrong to put "a" before protecao and doencas?