An old oversight with disease/var/viable_mobtypes.

code/datums/diseases/_MobProcs.dm

@@ -21,7 +21,7 @@
 		return FALSE
 
 
-	if(!(type in D.viable_mobtypes))
+	if(!D.infectable_mobtypes[type])
 		return FALSE
 
 	return TRUE

code/datums/diseases/_disease.dm

@@ -18,7 +18,6 @@
 	var/stage_prob = 4
 
 	//Other
-	var/list/viable_mobtypes = list() //typepaths of viable mobs
 	var/mob/living/carbon/affected_mob = null
 	var/list/cures = list() //list of cures if the disease has the CURABLE flag, these are reagent ids
 	var/infectivity = 65
@@ -30,10 +29,17 @@
 	var/list/required_organs = list()
 	var/needs_all_cures = TRUE
 	var/list/strain_data = list() //dna_spread special bullshit
+	var/list/viable_mobtypes //typecache of viable mobs passed down the below variable.
+	var/list/infectable_mobtypes = list()
 	var/infectable_biotypes = MOB_ORGANIC //if the disease can spread on organics, synthetics, or undead
 	var/process_dead = FALSE //if this ticks while the host is dead
 	var/copy_type = null //if this is null, copies will use the type of the instance being copied
 
+/datum/disease/New()
+	..()
+	if(viable_mobtypes)
+		infectable_mobtypes = typecacheof(viable_mobtypes)
+
 /datum/disease/Destroy()
 	. = ..()
 	if(affected_mob)