Learn Swedish: Vaccinet skyddar människan från sjukdomen. - The vaccine protects humans from the disease.

This is the categorical use of the definite singular. In Swedish, you often use the singular definite form to represent an entire species or class. While literally meaning "the human", it translates to "humans" or "mankind" in English.

Why not use the plural människorna if we mean humans in general?

Using människorna refers to a specific group of people, like "the people in this town". When referring to the human race as a biological species or collective whole, Swedish relies on the singular människan.

Are vaccinet and sjukdomen also examples of this categorical rule?

Yes! In a general statement like this, they represent the abstract concepts of the vaccine and the disease. Swedish frequently uses this definite singular pattern for scientific or medical facts, whereas English often defaults to plurals like "vaccines".

Can I write this sentence using plural indefinite nouns instead, like Vaccin skyddar människor från sjukdomar?

Yes, that is grammatically correct and a perfectly natural way to say "Vaccines protect people from diseases." However, using the definite singular for all three nouns gives this specific sentence a slightly more formal, scientific, or encyclopedic tone.