Learn Dutch: Zieken en zwakken worden in de massa vaak genegeerd. - The sick and the weak are often ignored in the masses.

In Dutch, you can turn an adjective into a noun. Because ziek (sick) and zwak (weak) refer specifically to people in the plural in this sentence, they get the -en ending.

If this sentence were about sick animals, would we still use zieken?

No. The independent plural suffix -n (which makes these adjectives end in -en) is strictly for human referents. For animals or objects, you would drop the -n and just use the -e ending, though it is usually more natural to explicitly say zieke dieren.

Why does this sentence use worden instead of zijn for the passive voice?

This sentence describes a general, ongoing action (they are ignored), which requires worden. If you used zijn, it would mean the action was already completed and you are describing the resulting state (they have been ignored).

Why is massa singular in Dutch but translated as plural in English?

De massa is a collective noun in Dutch. It literally means the mass or the crowd, but when talking about the general population in society, English prefers the plural translation the masses.