The UN Humanitarian Coordinator in Yemen, Lisa Grande, is quoted in several places (e.g. this CBS news article as saying (emphasis mine):
[SARS-CoV-2] is now in Yemen and may quickly spread. […] The factors are all here: Low levels of general immunity, high levels of acute vulnerability, and a fragile, overwhelmed health system.
Another phrasing (possibly a paraphrase of the same statement) is given in this short Guardian article:
The WHO has said it fears Covid-19 could rip through Yemen as the population has some of the lowest levels of immunity to disease compared with other countries.
These statements are a little ambiguous — they obvious reading is as meaning “immunity to diseases in general” but they could conceivably also be meant as “immunity to a specific disease, in the population in general”. But as I understand, no populations are believed to have specific pre-existing immunity to SARS-CoV-2/Covid-19 — or at least, the idea that existing immunity to other coronaviruses may confer some immunity to SACS-CoV-2 is quite tentative and not yet widely accepted as a significant effect. So the quote presumably is meant in the sense of “immunity to diseases in general”.
But what is this “general immunity”? All the explanations of immunity I’ve seem treat the concept of immunity as essentially disease-specific. Does it mean high rates of conditions like AIDS that reduce immune system function? Or is there some other sense in which people/populations can have higher or lower “general immunity”?