In fact, if your host lives for as long as possible without suffering any negative effects from your freeloading, is preferred by mates and most loved by peers, etc., that's when you spread the furthest.
Seeing it as a nefarious plot to kill as many as possible (by prolonging incubation periods, etc.) is humans thinking in terms of how their species behaved in the past. In fact there is only maladjustment.
Ah, you mean toxoplasma gondii. Heh.
Worth noting, seeing as it evolved to survive by cats and rodents, it is probably accidentally well-adjusted to humans (luckily we are not super different from rodents I guess). If it was as infectious as flu and had similarly mild effects it has now I'm sure most would be having it.
I wonder if some of the "worse" diseases are similarly mild to their animal hosts (birds and such), and us humans are just not as lucky to be similar enough in relevant aspects as with rodents and toxoplasma.
You may be oversimplifying here... consider that the host has an immune system and the infection has a limited time to either live in the host or to kill the host and remain in an infectious corpse for a while longer afterward.
As to corpses, fair enough. Ah, if only these infections caught up that humans don't often just leave bodies around...
But, along the road to perfect is good enough: If you can get along the silk road, you're doing ok.
Plague looks to have hit the spot: it does kill, and it does cause horrific sequelae, but it also survives long enough to spread.
Bubonic/Pneumonic expression probably helps, two forms means two vector-paths. Host of rats/fleas, well rats are pretty hardy, adaptable, and good at both hiding and breeding rapidly. So they can carry long distances in trade goods, and then bust out to exploit local food sources. Or, just move alongside the trade goods by chance, following the inadequate hygene of the road/river camps. (obviously, hiding on boats also works but the silk road was predominantly a land route)
Nothing those pesky humans did was going to interfere until two things happened: they learned some basic hygene, and they learned a couple of low-bar tricks to immunise, and then finally they invented antibiotics.
But I probably have a trick or two up my sleeve to fight back. Lets start with "helping" them make antibiotics less reliable, and then lets think about other strategies..