Is it true that during medieval times people were constantly disease ridden and that it was normal to have people walk around with lumps and rashes all over?

Considering the variety of treatments offered to us by Bald's Leechbook, I'm inclined to say "noooooooope". This, of course, does not cover the entirety of the Medieval Period, but I am reasonably secure in extending that 'nope', because this sounds like typical 'medieval medicine bad' myth. To which these previous answers address the question:

u/BedsideRounds has a non-exhaustive list of Stuff That Worked;
u/sunagainstgold talks about medicine and its efficacy, as well as medical education in the Late Middle Ages;

But the star of this post is u/BRIStoneman drawing from the 9th Century medical text Bald's Leechbook:

On the book's attitude to the treatment of medical conditions in general;
On the treatment of infected wounds;
On its recommendations for cuts and fevers (with bonus literacy downthread);
An assortment of treatments demonstrating that the leeches of Early Medieval England were on to something;
On its treatments for worms of all kinds.

Bonus: Bald's Leechbook is available in all its digitised glory here.

As ever, more insight is welcome from anyone with the knowledge, so if you'd like to type up a post of your own, please don't let this one stop you!