This sentence appears in Units > Verbs in the Infinitive 2 > Lesson 2: Using the Impersonal Infinitive,
«Fumar faz mal à saúde.»
It is translated as,
“Smoking is harmful to your health.”
But isn’t the correct translation,
“To smoke is harmful to your health?”
Wouldn’t it be more correct, in order to correspond to the first translation, to say,
«O fumar faz mal à saúde?»
Good question. Fumar is still classified as a verb form here (impersonal infinitive), not as a noun. Adding a definite article before it would promote a recategorization, but wouldn’t be idiomatic in Portuguese in this context.
On the other hand, in English, it is more idiomatic to say Smoking here than To smoke. So, in terms of idiomatic usage patterns in each language, Smoking is the best translation here overall and it’s not too confusing, from our perspective.
This balance between idiomatic vs. literal translations is very dynamic across our platform, because at times, one or the other might be more beneficial. Since we’re always open to reassessing cases like this, don’t hesitate to keep bringing our attention to them 
This exercise was confusing to me, because what looked like a verb was translated as a noun. This illustrates that, the extent to which you insist on expressing the languages idiomatically will parallel the extent to which we will have difficulty in learning Portuguese. When learning a new language, the last thing one learns are the idioms, because there are no rules to assist. Additionally, let me point out that the use of idioms is very personal and subjective. Trying to acquire someone else’s personal tastes in language usage simply puts another level of complexity on learning the language. If you must use idioms, then I would encourage you to always explain them with the non-idiomatic, grammatically accurate alternative.