Mostly, this depends on how intelligent the Reanimator is, and whether or not they have the support of the local people. If it's townsfolk vs. Virus Zombies vs. Reanimator, the result is fairly obvious: The Reanimator, if they're intelligent, will help the townspeople tide over the main attacks by the Virus Zombies, and after the Virus Zombies are broken, pick up all of the dead and take out the living. Reanimated Zombies have a huge benefit in their favor in that they're not a food source for the Virus Zombies; they're not living, they can't be contaminated and raised as a Virus Zombie, and they can be directed by the main Necromancer. If the townspeople have walls with stakes on them, and some undead to guard them, they should tide out the Virus Zombies in short order as they run out of food. Alternatively, the necromancer could create a Reanimated Zombie perimeter around the invested zone, to act as a quarantine, and starve them out. Since the Reanimated Zombies aren't food sources, don't need to break from vigil, and probably have/will soon have structures backing up their moves and a keen intelligence behind them, they will probably win out the long game against the Virus Zombies.
If the Reanimated Zombies are against the Virus Zombies and the townspeople, though, they have a huge advantage in the form of a strategist behind them. He can direct hordes to keep at the walls as normal, while also supervising underhanded tactics the humans/Virals aren't aware of, such as tunneling under the town to raid their catacombs/ossuary, and then striking in the center of the town. They win out the long game there for sure.
However, when you assume a mobile, advanced adventuring party, things get more dicey. The adventuring party is at a huge disadvantage towards the Viral Zombies, as they don't have the numbers to really crack down on Viral Population, and each hit they incur has a chance at defeating them. However, they have a decided advantage against the Reanimated Zombies, as the Reanimated Zombies have a central nervous node to strike down: The Necromancer. They can track him down through recent movements/sources of fresh troops, and no matter how many countermeasures he has, they will all eventually fail against a properly powerful party that can take him out. Traps only work once, he'll eventually recourse to reusing tactics, the party can know his strategic objectives, and so on. Of course, the problem with doing so is that he's the one keeping the Virals in check, and if the party captures him before his objectives are complete and hold him captive until he wipes out the virals, it's either a Xanatos Gambit, or he's going to get himself free before his use has run out (probably while also furthering his plans for taking over the city). And of course, by the time the Viral Zombies are dead and his use would otherwise have ran out, he'll have finished his primary objective (probably in the same masterstroke).
As a summation: The Necromancer is the key player in this infestation. He has the numbers to support the townsfolk and contain the Virals, and run well, he can basically play the war out perfectly to his advantage. The heroes are unable to touch him due to the fact that he has more or less unlimited resources and is the only one that can really contain the Viral Zombies, the Viral Zombies can't touch him because of the horde he has that they can't feed on, and the townsfolk are too pinned down by the Viral Zombies to do anything about the Necromancer that seems to be working with them against the Virals. Run well, he can present huge moral dilemmas for the players if they're opposed to him, or be the lynchpin that the players have to defend from opposing factions if they want to tide out the horde. If the Necromancer is an artifact, that artifact is the MacGuffin that will win the war; if it's a person, if they're working with the players they're a Living MacGuffin, if they're opposing the players, they're a Chessmaster who has contingencies for their defeat/capture.
Needless to say, I'm planning on a Necromancer Victory, save for the actions of an extremely competent adventuring party. And even then...