Dax's first host was Lela, with whom it joined in the early twenty first century. Lela became one of the Trill home worlds leading politicians1, and remained joined to the symbiont for nearly a century before finally dying in 2135. The next host was Tobin1; a brilliant engineer and mathematician, Tobin discovered one of the most original known proofs of Fermat's last theorem.1 He was an introverted and deeply shy individual who never had much success with relationships2, but he also carried the Dax symbiont for nearly a century before his death in 2230.
Dax was then transferred to Emony, a professional gymnast.1 Emony met with Lenoard McCoy shortly before he entered Starfleet medical school, and the two had a brief affair.3 In 2245 Emony died, passing the symbiont on to Audrid. Initially a scientist specializing in Trill physiology, Audrid eventually became an administrator and rose to the head of the Symbiosis Commission, one of the most renowned positions in Trill society. She died in 2284,1 Dax's last female host for over eighty years.4
After Audrid's death the Dax symbiont was joined with Torias, a test pilot. Something of a daredevil, Torias was only joined for a few months before he was killed in a shuttle accident. The Dax symbiont was subsequently placed into Joran, a musician. However, Joran proved to be mentally unstable and was responsible for several murders while joined. The fact that such an unstable mind was able to successfully join with a symbiont would have proved to the Trill population a fact that the symbiosis commission had secretly known for years; namely that more than half of all Trills are suitable for joining, rather than the tiny fraction generally claimed. Fearful that the relatively rare symbionts would become commodities to be bought and sold if this information leaked, the symbiosis commission placed a memory block in the mind of Dax, blanking out all knowledge of the Joran host, and joined it with Curzon.5
Curzon was one of the most well known of the Dax hosts; a Federation negotiator, he was very fond of the Klingon lifestyle and immersed himself in their traditions and culture - even becoming Godson to a Klingon child.6 He was a notable figure in the Khitomer peace conference of 2293, and went on to play a crucial role in many of the various treaties and alliances between the Federation and the Klingon Empire.
In 2373 Jadzia was murdered by Gul Dukat during his attempt to destroy the Bajoran Wormhole.7 The Dax symbiont was rushed back to Trill, but a medical emergency occurred during the voyage. Faced with the death of Dax if it failed to join with another host, the doctors talked an unprepared Trill, Ezri Tigan, into a joining she was unprepared for. The joining caused great confusion for Ezri, who was often unaware of her own age or gender, but she eventually settled down somewhat.8 Her family also found it difficult to accept the transition when Ezri returned home in 2375 to enlist their help in finding Chief O'Brien, who had vanished on their planet whilst investigating the Orion Syndicate.9
A Starfleet officer, Jadzia was assigned to Deep Space Nine as science officer in 2369.10 She served there for five years, participating in many of the important events in which the station was involved.11 Her notable achievements included helping then Commander Sisko in discovering the Bajoran wormhole, the first completely stable wormhole known to exist.10
In 2371 the memory block imposed on Dax by the symbiosis commission began to degrade, and the memories of Joran began to reassert themselves. Under the guise of treating hallucinations, Doctor Renhol of the symbiosis commission attempted to murder Jadzia in order to keep the secret but Captain Sisko was able to uncover the truth and threatened Renhol with exposure if Jadzia didn't survive.5 Jadzia subsequently decided to keep the fact that most Trills where eligible for joining a secret in order to preserve Trill society.
As Ezri, Dax remained on Deep Space Nine as a counselor.12 She eventually switched onto the command track and became third officer on the USS Aventine. After an attack on the ship killed both the Captain and First Officer, Ezri took command of the ship and successfully resolved the crisis. She was subsequently placed in permanent command of the ship.13
The Aventine played a major role in the Borg invasion of 2381. Ezri took the ship to the Delta Quadrant to investigate the wreck of the NX-02, theorising that the subspace tunnel network which transported the ship there was how the Borg were reaching the Alpha Quadrant.13 She convinced Captain Picard not to destroy the tunnel network14, and instead worked with Captain Riker of the USS Titan to penetrate the Borg collective and convince the Caeliar to destroy them.15
Like many joined Trill, Curzon served as a mentor in the initiate program where he was assigned to Jadzia. Curzon was secretly in love with Jadzia, and fearing that she would discover this and reject him he forced her out of the programme altogether. Jadzia later reapplied and successfully completed the initiate programme, the first person to do so after being rejected. Regretting his actions, Curzon later requested that the Dax symbiont be passed on to Jadzia when he died, and and this was done in 2367.16
Curzon Dax was a long time friend and mentor to Starfleet officer Ben Sisko. The two had a long friendship, and Sisko initially found it hard to get used to Dax being transferred into the much younger (not to mention female) Jadzia Dax.10
Jadzia began to develop romantic feelings for Worf Rozhenko after he was posted to Deep Space Nine. Although he was initially oblivious to these feelings, Jadzia eventually forced the issue and found that the attraction was mutual.17 The two became engaged at the outset of the Dominion war in 2373, as a way to give one another something to live for.18 The pair subsequently married in a Klingon ceremony on Deep Space Nine.19
Worf was devastated by Jadzia's death at the hands of Gul Dukat7, and concerned that her soul may not have reached Sto-Vo-Kor. He convinced Martok to achieve a great victory in Jadzia's name, destroying a Dominion shipyard at Monac IV. This, Worf believed, would ensure Jadzia's place in the afterlife.8
Ezri retained some of Jadzia's romantic feelings for Worf, but the two eventually decided to go their separate ways.20 She became involved with Doctor Bashir, and the two became a couple shortly before the end of the Dominion War.21