Common Cold in "Nasty, Brutish, &. . ."Edit
The common cold virus was engineered by the ancient Foitani before the Suicide Wars. A Foitan expedition had visited Earth and one of their researchers was killed by humans. (He or another member of his expedition apparently managed to abduct a male and female specimen.) The Foitani decided that humans were too violent and erratic to be allowed to develop into an advanced space-faring civilization, and so, per standard Foitan operating procedures, a respiratory virus was engineered which was supposed to be lethal, communicable, and incurable, and was introduced into the human population. The virus was indeed communicable and incurable but did not prove to be lethal, and it eventually came to be known as the common cold.
The Foitani did not revisit Earth before their empire was destroyed and their few surviving colonies were plunged into dark ages following the Suicide Wars. Millennia later, when spacefaring humans ranged as far as the Great Ones had at their height, modern Foitani who knew of their species' creation of the virus were sometimes perplexed and even a little offended that humans were not extinct.