You don't buy viruses. You evolve into them. After each Infect Game Mode run you pick 1 of 3 mutations for the next generation (1 of 4 once you have a second virus), and some mutations turn the lineage into a different virus.
The Family Tree shows one lane per virus, with every generation your lineage has been, in unlock order.
Lanes top to bottom: Influenza A, Retrovirus, Halovirus, Filovirus
Locked lanes show the level that opens them
Generations get flu-style names such as A/Brisbane/12/27 (H3N2), or RTV/M:C/Lagos/12 for Retrovirus
Each generation shows its virus ringed and lit in its mutation's colour, and the ring gets thicker each time the mutation is inherited again
Tap a virus's name at the start of its lane for its card: how it plays (passive, signature ability and ultimate) and, on the Real World tab, what it is in real life
From the Codex
What your virus passes on to the next generation.
Fun fact: Inheriting the same strain again stacks it and makes it stronger.