Live table, let's say 4-8, 10 handed.
One early player limps, he is loose passive.
CO raises, he is semi-loose and does raise reasonably often preflop but not a maniac.
SB folds.
Hero has
Hero glances over and limper has 4 chips already in hand, so he will certainly call if we flat-call, but may fold to a raise. In other words, he may call or fold but no way he is raising.

Hero is getting therefore either 4:1 or 5:1 to call (we will ignore SB .5 bet since it will be immediately raked anyway.

Hero's action?

Follow up question:
What pocket pairs do we:
1) call with
2) three-bet
3) fold preflop

What about AJo? AJs? 78s?

This should be interesting as I see a big split on this, but this type of situation comes up all the time so let's talk about it.

3 players is pretty much the worst number for a medium pocket pair. I'd 3 it if I thought limper folds. I might 3 bet it even otherwise if he limps with real rubbish.

Meh.. in a live game, a limper is almost never gonna fold, i just call and hope to flop the joint, and/or an underflop, and/or an uncoordinated flop with one high card that isnt an ace or a king, then its CR the COs CB FTW...