Heartworm refers to a relationship or friendship that you can't get out of your head, which you thought had faded long ago but is still somehow alive and unfinished, like an abandoned campsite whose smoldering embers still have the power to start a forest fire.

“心虫”指让你难以忘怀的一段爱情或友情，你以为这段情早就过去了，但却时常想起，难以释怀。就像弃置的露营地里残留的火星，仍然有可能引发一场森林大火。

For example:

I want to settle down with someone but this heartworm of my ex kept disturbing me.

我很想找个人安顿下来，但是跟前任的感情一直挥之不去。