Re: It's difficult/impossible tell when you got it.

It's a rule of thumb. The CDC suggests that a HerpeSelect reading under 3.0 should be re-evaluated after a few weeks. Some of the folks at that site I suggested can probably be more definitive about that. It's a lot more active forum so you generally get answers pretty quickly there.

No, it doesn't keep increasing over time without limit. It's not any sort of index that can really be used, for example, to figure out how old the infection might be.

Glad to be of help.