I've actually been working on a total conversion mod along these lines. The zombies I have are set up like this:
[CREATURE:ZOMBIE]
[NAME:infected:infected:infested]
[TILE:'Z'][COLOR:3:0:0]
[DAMBLOCK:1]
[EXTRACT:the plague:7:0:0]
[EXTRACT_VALUE:100]
[EXTRACT_PARALYZE]
[EXTRACT_PERMANENT]
[NOT_BUTCHERABLE]
[CAN_CIV]
[SLOW_LEARNER]
[CANOPENDOORS]
[BUILDINGDESTROYER:1]
[GRASSTRAMPLE:50]
[BODY:HUMANOID:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THROAT:NECK:SPINE:BRAIN:5FINGERS:5TOES:MOUTH]
[SIZE:7]
[MAXAGE:1000:10000]
[ATTACK:MAIN:BYTYPE:MOUTH:bite:bites:3:6:GORE][ATTACKFLAG_CANLATCH][SPECIALATTACK_INJECT_EXTRACT:1:10]
[ATTACK:SECOND:BYTYPE:GRASP:claw:claws:2:4:GORE][ATTACKFLAG_CANLATCH]
[CHILD:2][BABY:1]
[liTTERSIZE:5:10]
[FAT:0]
[EQUIPS]
[ALL_ACTIVE]
[EXTRAVISION]
[NO_DRINK]
[NO_EAT]
[NO_SLEEP]
[NOBREATHE]
[NOBLEED]
[NOEMOTION]
[NOEXERT]
[NOFEAR]
[NOSTUN]
[NONAUSEA]
[NOPAIN]
[PARALYZEIMMUNE]
[SPEECH_MALE:human_male.txt]
[STANDARD_FLESH]
[HOMEOTHERM:10067]
[LAYERING:10]
[SPEED:3000]
They shamble at an appropriately slow pace and, while you can hack off limbs to your hearts content, are very hard to kill in close combat and can take a few shot gun shells unless you happen to get the head. Since they can't speak (replace INTELLIGENT with CAN_CIV and CAN_LEARN (or SLOW_LEARNER)) they are guaranteed to be at war with everybody.
Speaking of which:
[ENTITY:EVIL]
[CREATURE:ZOMBIE]
[trANSLATION:HUMAN]
[ARMOR:ITEM_ARMOR_CLOAK:COMMON]
[ARMOR:ITEM_ARMOR_TUNIC:COMMON]
[ARMOR:ITEM_ARMOR_ROBE:COMMON]
[HELM:ITEM_HELM_HELM:COMMON]
[HELM:ITEM_HELM_CAP:COMMON]
[HELM:ITEM_HELM_HOOD:COMMON]
[HELM:ITEM_HELM_MASK:UNCOMMON]
[GLOVES:ITEM_GLOVES_GAUNTLETS:COMMON]
[GLOVES:ITEM_GLOVES_GLOVES:COMMON]
[SHOES:ITEM_SHOES_SHOES:COMMON]
[SHOES:ITEM_SHOES_BOOTS:COMMON]
[SHOES:ITEM_SHOES_BOOTS_LOW:COMMON]
[SHOES:ITEM_SHOES_SANDAL:COMMON]
[SHOES:ITEM_SHOES_SOCKS:COMMON]
[PANTS:ITEM_PANTS_PANTS:COMMON]
[PANTS:ITEM_PANTS_GREAVES:COMMON]
[PANTS:ITEM_PANTS_LEGGINGS:COMMON]
[PANTS:ITEM_PANTS_LOINCLOTH:COMMON]
[CLOTHING]
[SUBTERRANEAN_CLOTHING]
[CURRENCY:COPPER:1]
[CURRENCY:SILVER:5]
[CURRENCY:GOLD:15]
[SELECT_SYMBOL:WAR:NAME_WAR]
[SUBSELECT_SYMBOL:WAR:VIOLENT]
[SELECT_SYMBOL:BATTLE:NAME_BATTLE]
[SUBSELECT_SYMBOL:BATTLE:VIOLENT]
[SELECT_SYMBOL:SIEGE:NAME_SIEGE]
[SUBSELECT_SYMBOL:SIEGE:VIOLENT]
[SELECT_SYMBOL:ROAD:NAME_ROAD]
[SELECT_SYMBOL:TUNNEL:NAME_TUNNEL]
[SELECT_SYMBOL:BRIDGE:NAME_BRIDGE]
[SELECT_SYMBOL:WALL:NAME_WALL]
[SELECT_SYMBOL:REMAINING:DEATH]
[CULL_SYMBOL:ALL:DOMESTIC]
[CULL_SYMBOL:ALL:FLOWERY]
[CULL_SYMBOL:ALL:HOLY]
[CULL_SYMBOL:ALL:PEACE]
[CULL_SYMBOL:ALL:NEGATOR]
[CULL_SYMBOL:ALL:GOOD]
[STONE_PREF]
[USE_EVIL_ANIMALS]
[USE_EVIL_PLANTS]
[USE_EVIL_WOOD]
[ABUSE_BODIES]
[SPHERE_ALIGNMENT:DEATH:512]
[ART_FACET_MODIFIER:GOOD:0]
[ART_FACET_MODIFIER:EVIL:512]
[FRIENDLY_COLOR:5:0:1]
[DEFAULT_SITE_TYPE:CITY]
[liKES_SITE:CITY]
[liKES_SITE:DARK_FORTRESS]
[liKES_SITE:CAVE_DETAILED]
[CAN_HAVE_MILITARY_SITE_LEADER]
[START_BIOME:ANY_DESERT]
[START_BIOME:ANY_SAVANNA]
[START_BIOME:ANY_WETLAND]
[BIOME_SUPPORT:ANY_WETLAND:4]
[BIOME_SUPPORT:ANY_DESERT:5]
[BIOME_SUPPORT:ANY_FOREST:4]
[BIOME_SUPPORT:ANY_OCEAN:2]
[BIOME_SUPPORT:ANY_LAKE:2]
[BIOME_SUPPORT:MOUNTAIN:2]
[BIOME_SUPPORT:ANY_GRASSLAND:4]
[BIOME_SUPPORT:ANY_SAVANNA:4]
[BIOME_SUPPORT:ANY_SHRUBLAND:3]
[BIOME_SUPPORT:ANY_RIVER:2]
[PROGRESS_TRIGGER_POPULATION:1]
[PROGRESS_TRIGGER_PRODUCTION:1]
[PROGRESS_TRIGGER_TRADE:1]
[ACTIVE_SEASON:SPRING]
[ACTIVE_SEASON:SUMMER]
[ACTIVE_SEASON:AUTUMN]
[ACTIVE_SEASON:WINTER]
[MAX_STARTING_CIV_NUMBER:2]
[START_GROUP_NUMBER:30]
[MAX_POP_NUMBER:1000000]
[MAX_SITE_POP_NUMBER:150]
[RELIGION:ANY_APPROPRIATE_POWER]
[BEAST_HUNTER]
[SCOUT]
[PERMITTED_JOB:MINER]
[PERMITTED_JOB:CARPENTER]
[PERMITTED_JOB:BOWYER]
[PERMITTED_JOB:WOODCUTTER]
[PERMITTED_JOB:ENGRAVER]
[PERMITTED_JOB:MASON]
[PERMITTED_JOB:ANIMAL_CARETAKER]
[PERMITTED_JOB:ANIMAL_TRAINER]
[PERMITTED_JOB:HUNTER]
[PERMITTED_JOB:TRAPPER]
[PERMITTED_JOB:ANIMAL_DISSECTOR]
[PERMITTED_JOB:FURNACE_OPERATOR]
[PERMITTED_JOB:WEAPONSMITH]
[PERMITTED_JOB:ARMORER]
[PERMITTED_JOB:BLACKSMITH]
[PERMITTED_JOB:METALCRAFTER]
[PERMITTED_JOB:GEM_CUTTER]
[PERMITTED_JOB:GEM_SETTER]
[PERMITTED_JOB:WOODCRAFTER]
[PERMITTED_JOB:STONECRAFTER]
[PERMITTED_JOB:LEATHERWORKER]
[PERMITTED_JOB:BONE_CARVER]
[PERMITTED_JOB:WEAVER]
[PERMITTED_JOB:CLOTHIER]
[PERMITTED_JOB:GLASSMAKER]
[PERMITTED_JOB:FISHERMAN]
[PERMITTED_JOB:FISH_DISSECTOR]
[PERMITTED_JOB:FISH_CLEANER]
[PERMITTED_JOB:CHEESE_MAKER]
[PERMITTED_JOB:MILKER]
[PERMITTED_JOB:COOK]
[PERMITTED_JOB:THRESHER]
[PERMITTED_JOB:MILLER]
[PERMITTED_JOB:BUTCHER]
[PERMITTED_JOB:TANNER]
[PERMITTED_JOB:DYER]
[PERMITTED_JOB:PLANTER]
[PERMITTED_JOB:HERBALIST]
[PERMITTED_JOB:BREWER]
[PERMITTED_JOB:SOAP_MAKER]
[PERMITTED_JOB:POTASH_MAKER]
[PERMITTED_JOB:LYE_MAKER]
[PERMITTED_JOB:WOOD_BURNER]
[PERMITTED_JOB:MECHANIC]
[PERMITTED_JOB:SIEGE_ENGINEER]
[PERMITTED_JOB:SIEGE_OPERATOR]
[PERMITTED_JOB:PUMP_OPERATOR]
[PERMITTED_JOB:CLERK]
[PERMITTED_JOB:ADMINISTRATOR]
[PERMITTED_JOB:TRADER]
[PERMITTED_JOB:ARCHITECT]
[ETHIC:KILL_ENTITY_MEMBER:UNTHINKABLE]
[ETHIC:KILL_NEUTRAL:ACCEPTABLE]
[ETHIC:KILL_ENEMY:ACCEPTABLE]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:UNTHINKABLE]
[ETHIC:TORTURE_AS_EXAMPLE:UNTHINKABLE]
[ETHIC:TORTURE_FOR_INFORMATION:UNTHINKABLE]
[ETHIC:TORTURE_FOR_FUN:UNTHINKABLE]
[ETHIC:TORTURE_ANIMALS:UNTHINKABLE]
[ETHIC:TREASON:UNTHINKABLE]
[ETHIC:OATH_BREAKING:UNTHINKABLE]
[ETHIC:LYING:UNTHINKABLE]
[ETHIC:VANDALISM:ACCEPTABLE]
[ETHIC:TRESPASSING:ACCEPTABLE]
[ETHIC:THEFT:UNTHINKABLE]
[ETHIC:ASSAULT:ACCEPTABLE]
[ETHIC:SLAVERY:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_OTHER:ACCEPTABLE]
[ETHIC:EAT_SAPIENT_KILL:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:UNTHINKABLE]
With this set up, although they start of with only two sites (isolated outbreaks) they tend to take out most human civs (and I upped them to twelve starting civs a piece) and make it the Infested Age in under two hundred years.
Getting killed by these things is hilarious, since they're almost never alone. One bites you, and you try to kill it before the plague sets it, then another shows, and another, and pretty soon there's a dozen all tearing at your internal organs until you eventually bleed out. Then they get up and shamble off in random directions.