I've read this many times since the age of 11, and always thought it to be a socially relevant story on AIDS, drug use and loving relationships.
Unfortunately looking at it now, there are some deep flaws with a few elements of the story. First off, it's from a completely hetero-normative perspective. How it affected the gay population in the early 90s is hardly mentioned at all.
It's also insidiously bi-phobic - with the beautiful blonde straight protagonist's life being completely shattered by her sleeping with a bisexual man early in her teens (not using a condom in a one night stand) Though he's not made out to be an outright villain - it still serves as an unjust warning that straight women shouldn't have sex with bisexual men.
Or else they end up like poor Jo, sick and eventually dead from AIDS.