Background
The parasites are a relative newcomer to interstellar travel. Their Bio-ships have been sublight capable for more than seven hundred years. They do not have a name for themselves, but have considered themselves to be the highest form of life. This attitude was reinforced by the first colonists that tried to settle their planet. These Colonists called themselves Draconins, the parasites simply called them the firsts. The colony had been overrun in a matter of days, although a considerable amount of time was spent attempting to digest the new information that these Draconins possesed. A talent that they eventually improved upon in regards to speed. By the end of the 23rd Century they began using the Draconin colonists to refit their bioships for FTL travel. In 2307 the parasites came across the Draconin homeworld, had the Draconins shot first and asked questions later, the parasite invasion force would have been eliminated, as the Bioships had no weapons and once the Draconin fleet figured out what was going on, thirteen of the bioships were killed before the fleet came under fire from their own planetary defense network. Rather than stay and be destroyed by thier own defense grid, the Draconin fleet withdrew to one of its outer colonies. The parasites realized that the humanoids, while primative, were extremely capable of defending themselves, and began to refit their bioships again with Draconin Disrupter technology. The Battle of Draconin II in 2309 saw the end of the Draconin civilization as an independant nation. But the losses to the parasite's fleet was enormous, eighty percent of their bioships were killed by the Draconin defense net. New ships that were born to replace those lost gave rise to distinctly different classes of Bioships.
First Contact with the Federation
In 2347, a Federation Long-Range Explorer, USS Ferdinand Magellan, NCC-21357, a Venture Class Scout Cruiser, entered the Draconin II system. The debries from the battle in 2309 was still floating around the system. The Magellan noted no life signs on the primary planet, and so assumed that this was the home of a destroyed civilization and sent down an away team. The humanoids who had not been killed defending the planet from the parasites were infected and killed during an accident that wiped out the only major city on the planet. It did not take long for the Parasites on the planet to overwhelm the Magellan Landing Party and eventually take over the ship.
Enlightenment
Once the Magellan crew has been absorbed, the parasites found themselves with a wealth of knowledge, but assimilating the knowledge took time, and the Federation believed the Magellan was lost with all hands, not unusual for long range explorers, but not commonplace either. When the Magellan finally returned in 2362, the crew claimed to know nothing of the time lost. They were welcomed home by several of the top brass of Starfleet as heroes. The Parasites used the party as their opportunity to sieze control of Starfleet Command.
Invasion Thwarted
To prevent another Draconin II, the Parasites began issuing orders to Federation Starships to ignore key sectors of Federation Space, those few that caught on, were either infected or eliminated through sabotage. The intervention of the Galaxy Class USS Enterprise NCC-1701-D stopped the invasion with the Bio-ships marshalling for the Invasion. A signal was recieved at the expected time, but it contianed something unexpected...an Abort signal.
Waiting in the Wings
Since their first aborted attempt to take over the Federation, they wanted it. They were patient, they had three opportunities to get back into the Federation, the first took place only a few years after thier aborted attempt, following the decimation of the Federation Starfleet at the Battle of Wolf 359. But the speed with which Starfleet rebuilt following that disaster cancelled the plan. The second opening took place after the Battle of Sector 001 in 2373. While more devastating than the Battle of Wofl 359 in terms of ships lost, the Federation defenses were still strong enough to prevent a full scale operation. Finally the Dominion war occured, and they siezed that opportunity to infiltrate Starfleet during its weakest moments. With Federation and Klingon ships rapdily being stripped from their mutual border to engage the Dominion forces, the Parasites sent a small Bio-Ship Frigate to attack and board a Federation Transport bound for Space Station K-7. The Parasites had their foothold, but decided to work slowly to avoid detection. By 2379 they had under their direct control, a full one-quarter of Starfleet, and had decided to make their move on the Admiralty. After succesfully infesting the Admiralty they enacted a bit of revenge, and destroyed the current vessel to bear the name Enterprise, a Soverign class Starship, lest she stop their plans again. The parasites did hit a bit of bad luck, Bajor's unexpectedly sudden decision to accept Federation Membership caught many of the Dominion War Veteran Admirals away from Fleet Command during the takeover, and they continue to cause havoc for the parasite controlled Starfleet.