What's the Name of That Book??? discussion

This Book was read in 2010, in a Highschool. These 2 kids, a boy and a girl who are friends/siblings? The girl is MEGA smart, ad she starts de-coding signals received from a wormhole of some sort. They try to figure out what it means, and the boys older brother, who is a rougher kind of guy, steps in to help. Turns out, the Cold (Or Flu) materializes in the form of a body and begins attacking the whole city. The best way for people to fight it is with water. At on point, they are all in suits at the bottom of a pool to stay away from these creatures. They ultimately seem to fail, but get in a sub-marine and find a cave. The books ends seeming to tell the reader that they will send messages to warn their past selves about the epidemic from the cave and hope that their other selves succeed. will update when details come to mind, thank you!

Could it be Feed by Mira Grant?

Unfortunately, No. Thanks for the input though Sarah. Feed looks like a good read to check out!

The Tomorrow Code by Brian Falkner

Yes! that is it! Thank you so much Juels!

You're welcome.