What is the infection counter in Dead Cells?

From a steamcommunity post:

It's actually understood. Getting hit by the elites in the special rooms in Castle infects you. After amassing too much infection, the player will take damage until either death or leaving the level.

So getting too much infection is not a good thing. Apparently they found the Dead Inside mutation too strong and nerfed it.

To expand after a comment:

• The Malaise was referred to as the infection before update 1.0.
  • Malaise works in a point system. The highest tier of infection is 10 points. At tier 10, The Beheaded takes massive damage.
  • The use of a healing charge takes off 3 tiers of Malaise.
  • The key elites in High Peak Castle apply 2 points of Malaise per hit. - - Activating 4 Boss Stem Cells results in every enemy applying a point of Malaise upon hit.

Straight from the wiki