Rare diseases can vary in prevalence between populations, so a disease that is rare in some populations may be common in others. This is especially true of genetic diseases and infectious diseases.

罕见病在不同的人群中的流行性差异很大，某种疾病在一个人群中很罕见，在另一个人群中可能就很常见了，这就是流行性疾病和遗传性疾病之间的差异所在。

youdao

Actually, what I want is very simple, but it's also very difficult to achieve. Maybe love is not rare, however true love is definitely rare.

或许正是这种念头支撑起了一种希望和等待。相信了，做到了，即使无果，也不会有太耿耿于怀的遗憾了。

youdao

Confirming Mister Muse's true age is difficult because birth documents are rare in Somalia.

VOA: special.2009.04.25

Determining Muse's true age is difficult because birth certificates are rare in Somalia,a country which has not had a functioning government for nearly two decades.

VOA: standard.2009.04.22

But as a rare, true three-shot par-five, it can easily peel off the weak of heart.

WSJ: 2012 U.S. Open: An Olympian Effort Just to Survive

Some have XX chromosomes in some cells of their body and XY in others those rare, true hermaphrodites.

FORBES: Is Sex Reassignment Surgery a Basic Human Right?

As rare as true love is the Argyle pink diamond.

FORBES: The $1 Million Pink 'Prosperity' Diamond