Learn Swedish: Sjukdomen dödade många människor.

Both mean "people", but människor refers to individuals or human beings in a countable sense (you can count "many humans"). Folk is an uncountable collective noun meaning "a crowd" or "the public" (as in "there were a lot of people at the concert"). Since we are talking about a quantifiable number of individual lives lost, många människor is the correct choice.

Could I use the verb dog instead of dödade in this sentence?

No, because dog is the past tense of dö (to die), which is an intransitive verb—meaning it happens to the subject. Döda means "to kill", which is a transitive verb that requires a direct object. Here, the disease is actively doing the killing to someone else, so you must use dödade.

Is there a trick to remember that sjukdom is an en-word (making it sjukdomen)?

Yes! Almost all Swedish nouns that end in the suffix -dom (like ungdom for youth, or rikedom for wealth) are en-words. This suffix is used to describe a state or condition, closely related to the English suffix "-dom" found in words like "kingdom" or "freedom".

How would I rewrite this sentence in the passive voice to say "Many people were killed by the disease"?