Harvesters were deadly nano-biogenic weapons.
Harvesters were developed several centuries prior to 2370 and originally took ten years to perfect. The harvester itself was an invisibly-small synthetic virus, tiny enough to enter the body through the skin, and lethal alone. It was classified as a biomechanical gene disruptor. The T'Lani and Kellerun each produced mass quantities of the weapon during their centuries-long war. The entire population of T'Lani III was devastated by harvesters.
When the two factions finally found peace, they collected their entire supply of harvesters and a team of scientists, led by Nydrom, set to the task of destroying the weapons. The harvesters were kept in transparent cylinders and appeared as an orange gel. The scientists had little success, so in 2370, both sides applied for Federation assistance to destroy the harvester cylinders. Julian Bashir and Miles O'Brien of Deep Space 9 were assigned to help neutralize the weapons. The harvesters were remarkably resistant to broad-spectrum radiation. After one week and 375 test sequences, Bashir finally found the right combination of muon frequencies to eliminate the gene disruption phasing of the weapon. When a harvester cylinder was neutralized, the chemical turned from orange to dark green.
When the harvesters had been destroyed, the T'Lani and Kellerun governments ordered that anyone with technical knowledge about the harvesters was to be killed so that the weapons could not be recreated. A Kellerun military team was dispatched to the T'Lani cruiser, the site of the harvester destruction effort, to assassinate the scientists working on the project. Bashir and O'Brien survived the attack, but O'Brien was accidentally exposed to the harvester virus.
While hiding out in the rubble of a city on T'Lani III, O'Brien started to suffer the symptoms of harvester infection. He had a high fever, and reported intense chills and blurry vision. When the two were rescued a day later, O'Brien was near death, but Bashir was able to save him when they returned to Deep Space 9. (DS9: "Armageddon Game")