Changes way viruses are stored in mobs.

Now it's an associative list:
uniqueID - virus datum

Updated most instances to match it.

code/game/gamemodes/epidemic/epidemic.dm

@@ -124,14 +124,15 @@
 
 	for(var/i = 0, i < lethal_amount, i++)
 		var/mob/living/carbon/human/H = pick(crew)
-		if(lethal in H.virus2)
+		if(lethal.uniqueID in H.virus2)
 			i--
 			continue
-		H.virus2 += lethal.getcopy()
+		H.virus2["[lethal.uniqueID]"] += lethal.getcopy()
 		infectees += H
 
 	var/mob/living/carbon/human/patient_zero = pick(infectees)
-	patient_zero.virus2.stage = 3
+	var/datum/disease2/disease/V = patient_zero.virus2["[lethal.uniqueID]"]
+	V.stage = 3
 
 	cruiser_arrival = world.time + (10 * 90 * 60)
 	stage = 1

code/game/objects/effects/decals/Cleanable/humans.dm

@@ -144,4 +144,4 @@
 	icon = 'blood.dmi'
 	icon_state = "mucus"
 	random_icon_states = list("mucus")
-	var/list/datum/disease2/disease/virus2 = null
+	var/list/datum/disease2/disease/virus2 = list()

code/game/turfs/simulated.dm

@@ -108,14 +108,14 @@
 		if(!B.blood_DNA[M.dna.unique_enzymes])
 			B.blood_DNA[M.dna.unique_enzymes] = M.dna.b_type
 		if (M.virus2.len)
-			B.virus2 |= M.virus2
+			B.virus2 |= virus_copylist(M.virus2)
 		return 1 //we bloodied the floor
 
 	//if there isn't a blood decal already, make one.
 	var/obj/effect/decal/cleanable/blood/newblood = new /obj/effect/decal/cleanable/blood(src)
 	newblood.blood_DNA[M.dna.unique_enzymes] = M.dna.b_type
 	if (M.virus2.len)
-		newblood.virus2 = M.virus2.Copy()
+		newblood.virus2 |= virus_copylist(M.virus2)
 	return 1 //we bloodied the floor
 
 
@@ -125,7 +125,7 @@
 		var/obj/effect/decal/cleanable/blood/this = new /obj/effect/decal/cleanable/blood(src)
 		this.blood_DNA[M.dna.unique_enzymes] = M.dna.b_type
 		if (M.virus2.len)
-			this.virus2 = M.virus2.Copy()
+			this.virus2 = virus_copylist(M.virus2)
 
 	else if( istype(M, /mob/living/carbon/alien ))
 		var/obj/effect/decal/cleanable/xenoblood/this = new /obj/effect/decal/cleanable/xenoblood(src)