As an additional info, i copied this from the F1 help file:
The Force Pestilence Level
This value represents a force’s vulnerability to disease losses, and is limited to the range 0 to 50. It sets a percentage of equipment lost by every unit in the force on every turn. Infantry, horse transport and cavalry equipment is lost at this rate, while all other types of equipment are lost at half the rate. Within each individual location, the losses are multiplied by:
/Pestilence Loss Multiplier
frozen, not snowy
hot, not mud or marsh
hot, mud or marsh
seaborne land unit
These multipliers are cumulative. The worst cases are unsupplied frozen, snowy or unsupplied hot muddy locations, with a multiplier of 1.5625. If the overall force pestilence value were set at 3%, losses in the worst case situations would actually be 4.69% for infantry, cavalry and horse transport equipment, and 2.34% for all other types of equipment.
Half of infantry, cavalry, and horse transport equipment lost by units in supplied locations is sent to the replacement pool. All non-infantry, non-cavalry, non-horse transport equipment lost by units in supplied locations is sent to the replacement pool. All other pestilence losses are permanent.
This value should rarely be set above 5%, and in most cases should be lower. The default is 0%. This value may be set from the OOB editor Edit > Modify Current Force menu, or by use of the Force Pestilence (Pestilence 1, or Pestilence 2) event effects.