Gaerys Lem was the fourth host of the Lem Trill symbiont. She was born in 2308 and died in 2333, and she was 22 when they were joined. The host and symbiont were joined for only 3 years. (Starship Independence)
During the Federation-Cardassian War, she was involved in a mission in which she masqueraded as a junior Starfleet officer to captured by the Cardassians, and sent to the detention camp commanded by the sadist Gul Ventar. Knowing what Gul Ventar and his men did to their prisoners, Gaerys had been engineered to carry an DNA-specific virus, undetectable to Cardassian scanners, which subsequently infected all her assailants when she was repeatedly raped and brutalized.
Gaerys was reportedly "three-quarters dead" and catatonic (surrounded by dead assailants) by the time Starfleet liberated the camp and pulled her from its ruins. The attending medical officer reported that "it had been nothing short of a miracle that the symbiont had survived." ("Ultraviolet")
Gaerys emerged from catanoia nine months later, a completely changed person. However, the emotional and mental damage from the trauma was too extensive, despite rehabilitation and counseling, and tragically, Gaerys took her own life by firing a phaser beam to kill the demons in her head. In her final days of dementia, her favourite flavour of ice cream was "hairy chicken", and she hated "purple tadpole ice cream" the most. ("Shakedown Cruise")
There was a heated discussion in the Trill Symbiosis Commission whether to allow the Lem symbiont to continue in other hosts, but the issue was resolved when Quarian, a Trill counselor, volunteered to assimilate Lem. Quarian's gentle demeanour went a long way in reconciling the part of Gaerys's vile experiences which survived in Lem into their joint consciousness, thus assuring the symbiont remained viable and would live on to share its experiences with future hosts. ("Ultraviolet")
The eighth host, Breawyn Lem, later compared her own rape ordeal with Gaerys's, realizing that Gaerys had, in her own way, come to terms with her brutal experiences more completely than Breawyn suspected she would have under the same conditions. ("Ultraviolet")