'At the edge of the Old Forest, south through the Barrow-downs, there are many bears suffering from a terrible plague. The only way I can see to save them from their agony is to destroy them and ensure that they do not spread this plague into the remainder of the Old Forest.
'You will find the bears and the plague-carries deep to the south where the walls of the Old Forest meet the Barrow-downs. Destroy the bears and then destroy the diseased crawlers infesting them.
'You may find some of the disease-carrying crawlers outside of the hosts as well.'
Bob Redthistle told you that the bears near the bounds of the Old Forest, deep within the southern Barrow-downs, are being devoured from within by barrow-crawlers.
Defeat angry bears (0/8)
Defeat large barrow-crawlers (0/24)
The bears and the disease-bearing crawlers can be found on the border of the southern Barrow-downs and Old Forest.
Bob Redthistle wants to destroy the diseased bears before they spread their plague throughout the Old Forest.
Talk to Bob Redthistle at the Dead Man's Perch
Bob Redthistle can be found at the Dead Man's Perch.
You should return to Bob Redthistle and tell him that the grim work he required is complete.
Both of these mobs are common in the very south-west corner of the Southern Barrow-downs. This is the same area as the Corrupted Trees for "From death, comes life" and "Unfortunate Harvest".