By the 24th century, the plague was wiped out, but the Brekkians exploited the situation by concealing this fact from the Ornarans, and they continued supplying them with the drug, to which they eventually became addicted. Generations of drug addiction and dependence resulted in a loss of intelligence to the Ornarans, which the Brekkians exploited even more.
By 2364, the Ornarans had lost their ability to transport the felicium from Brekka to Ornara. This put the economy of Brekka under threat, because they'd become dependent on the income from the sale of felicium to the Ornarans – and the sole remaining industry on Brekka is the production of the drug. (TNG: "Symbiosis")