Makes sense: the undocumented ones are afraid to seek medical help, lest they be turned in.

The other immigrants: does the government still screen for medical problems pre-arrival? This used to be a big thing in Ellis Island days.
In any case, by the time an immigrant refugee gets to the U.S., the length and difficulty of the journey, from their place of origin to the processing point/refugee camp to the U.S. has probably weeded out the really unwell. Only the fittest survive.
And, if they weren't used to being able to access medical care before, it's probably not their first thought now. Especially as it costs money.