The Srivani were a warp-capable species from the Delta Quadrant that believed experimentation on other species was an acceptable method of investigating their own physiology. They justified the inevitable harm caused by citing the number of Srivani lives saved as a result. (VOY: "Scientific Method", Star Trek: Phoenix-X)
Their normal method of experimentation involved using an interphase cloaking device to render themselves, their ships and their devices both invisible and intangible. They then attached to the hull of a passing ship and would spend several weeks carrying out experiments on the crew. These experiments range from changes in body chemistry to complex DNA manipulation carried out by cloaked tags, attached to a host's DNA.
In 2384, while investigating Borg transwarp conduits, the USS Phoenix-X was forced to into the Delta Quadrant where they were experimented on by the Srivani. As a result, Kayl, a human, was given telepathic power through the experimentations. Meanwhile, an infestation of interphasic organisms, from a previously installed Interphase cloaking device aboard the Phoenix-X, began to harm the Srivani (after some testing on the organisms themselves). The crew of the Phoenix-X were forced to help the Srivani and eliminate the interphasic organism infestation. (Star Trek: Phoenix-X: "Fantastic Method")