As PERCE_NEIGE said, it's implied. But let me add something that may help you understand a bit more:
In English, we can say:
- The doctor helps the sick.
- The doctor helps the poor.
- The doctor helps the rich.
- The doctor helps the Italian.
In all these sentences, someone might ask: The doctor helps the sick/poor/rich/Italian what?
The answer is that the doctor helps the sick/poor/rich people, and the Italian person.
That the doctor is helping people is understood, even though the word "person/people" doesn't appear. It's the same with Latin, but in Latin it's more common.
(And it happens a lot in Spanish, Italian, French, Portuguese, and many other languages too, I'm sure.)
There isn't a sentence discussion for single words, so this one seemed closest:
I had a picture of an obviously female doctor, so I wrote medica and was marked wrong and told it should have need medicus. Two questions before, I was going to write medicus, knocked the mouse over the hints, and saw medica, so put that and was marked correct.
Please could the single word be corrected to accept both ~us and ~a, or else use a male picture!
Reported as "My answer should have been accepted"