Cheapest priced copy (Lorwyn Eclipsed Commander): $0.69 at Card Kingdom, near mint, their price list as of .
Blowfly Infestation 
Enchantment · Uncommon · Shadowmoor · #58
Whenever a creature dies, if it had a -1/-1 counter on it, put a -1/-1 counter on target creature.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.69 to $37.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Lorwyn Eclipsed Commander | 46 | Uncommon | $0.69 | — |
| Shadowmoor | 58 | Uncommon | $1.29 | $37.99 |
2 rulings from Wizards of the Coast
- 1 May 2008When Blowfly Infestation's ability resolves, it will put a single -1/-1 counter on the targeted creature. It doesn't matter how many -1/-1 counters were on the creature that went to a graveyard as long as it had at least one on it.
- 1 May 2008This ability is mandatory. If you're the only player who controls any creatures when this ability triggers, you must target one of them.