[Inky]

Healing doesn't even say what the criteria are - it only says "from +1 for the common cold to -15 for AIDS".
That seems like it's not based purely on how difficult they are to cure by modern methods, since both the examples are virus diseases and difficult-though-not-impossible to make any difference to with modern medicine (it's just that with colds it doesn't matter). It could be based on severity (although there are plenty of things that, untreated, will kill you faster than AIDS), or on some vague combination of severity and how hard it would theoretically be to cure if you were in a TL8 world and close to medical supplies which you may not be, or it could just be an invitation to the GM to make it as easy or as difficult as would be useful to the plot.