The Space Giants

The main characters are Max and Sarah, two girls who had dreamed of going to space ever since they were kids. They go to the planet Zena and find it is inhabited by giants. The planet has an abundance of alien flora and fauna including trees and bushes that are crystalline and look almost like rock candy. The giants communicate in a language that sounds like clicking noises and is very rhythmic and almost sounds like a song. The planet has lakes that are radioactive and are highly toxic to humans, but the giants can drink from them without being harmed. After Max and Sarah find this out, they realize that the giants could hold the key to curing the "zenovirus" (Named after the planet Zena because Zena is known for it's irradiated lakes) that mysteriously popped up on Earth 3 years ago and has been decimating the population ever since. The Zenovirus is caused by irradiated water and causes a rapid mutation in people that makes them extremely violent and causes death in less than three days after being contracted. The girls are there on a colonization mission, being told to persuade the giants to give some of their land to the humans.