Someone (probably Hermione) finds an obscure spell that, when cast at an inferi, will cause them to hunt down the caster's enemies and kill them. The spell is irreversible and, once cast, they cannot harm anyone except the enemies of the caster.

She doesn't think anymore about it, since she wouldn't know where to try it out.

Later, however, Harry tells her about the lake full of inferi.

Unfortunately, inferi are no known for their subtly and the Muggle authorities jump into action, admittedly slowly.

However, these are not normal inferi, Voldemort cursed them so that anyone they killed also became an inferi… and so on… and so on.

The Muggle police, even their firearms units, and later the army don't prove particularly effective against these 'drugged-up cosplayers'. Bullets don't seem to hurt them and the British Army hasn't used flamethrowers since the 40s. Explosives somewhat work, but there's a limit to how many tanks and grenade launchers they have and there are thousands of these things marching across south-east England, tens of thousands. Nobody knows how many and the British haven't had to fight a war in England since the Civil War.

Normally, this is the kind of thing is something the Ministry would act on straight away. Unfortunately, the day that they appeared was the same day that Voldemort's new government purged the unfaithful from its ranks. By the time someone important knew how critical the situation was, it was already too late.

Two weeks later, in the Forest of Dean...

Hermione looked up as another group of jet planes shot by overhead. That was the third time this day. Were they near an RAF base or something?

"I hope Harry gets back soon," Ron said. "I'm starving."

"Here I am," Harry said, ducking through the tent flap. "Guys, you have to see this."

He slammed a newspaper on the kitchen table.

"Infection reaches London," Hermione read. "What infection?"

Harry began rifling through the pages.

"The Muggles are calling it the Zed-virus, but to me, it looks an awful lot like…"

"Inferi," Hermione said, looking at the pictures in the paper. Her hands began to shake. "How many…?" She couldn't get the words out.

Harry grimaced.

"Three million already."

They had to give Hermione their last Calming Draught to stop her screaming.

"What happened?" she said, her voice hoarse.

"The cave wasn't too far from Portsmouth," said Harry. "As far as I've been able to tell, the Inferi spread from there, turning hundred of Muggles before their authorities knew anything was happening."

"Wait," Ron interrupted. "Inferi don't work like that. They kill people but don't change them into more Inferi. What idiot wizard would create a self-replicating dark creature?"

"The Dark Lord, obviously," Hermione said. "But he didn't expect them to leave the cave. He just wanted them to bolster the defences with fresh bodies, not spread across the country."

"Well, they have," Harry said. "They're as far as Devon, Bath and now London. The Muggles have mobilised, well, everything they have, but guns don't have much effect and the regular army is too small to contain millions of the things. That's what tipped me off that something was wrong. A couple of old guys grabbed me and asked me why I wasn't headed to the front. I had to confund them to get away."

"Conscription?" Hermione asked weakly.

"Yeah."

She was now extra glad that her parents were in Australia.

"But this wasn't supposed to happen!" she said loudly. "They're only meant to go after him!"

"Maybe if a Muggle tried to stop them, they'd attack in order to continue their mission," Harry suggested. "Maybe we just cast the spell wrong."

Hermione held her head in her hands.

"Are they evacuating the whole country?" she asked.

Harry grimaced again.

"They can't." He turned the newspaper to a different page. "Ten thousand Muggles set off in boats to try to get to France, but the French Navy…"

Hermione stared at the gory colour photographs. The sea was full of destroyed boats and floating corpses, all washed red with blood.

"No one is risking letting the 'virus' out of Britain. The Americans are sending tons of weapons and there are thousands of their fighter jets helping with the bombing campaign. It's slowing the Inferi down, but they're not stopping."

"What are the Ministry doing?" asked Ron.

"I snuck into Diagon Alley," Harry said.

"Harry!", "That was too dangerous!"

"I had to find out, okay?!" He pulled the Daily Prophet out of his pocket. "It's all Death Eater propaganda, but if you read between the lines, it's clear that they have no grip on the situation at all. The ICW is screaming at them. Wizards and witches across the country are panicking. The senior Death Eaters are all quiet. They're blaming it on 'Mudblood terrorists'."

Hermione choked out a laugh.

"Well, they're right!" she said hysterically.

I'm not sure where you'd take it from here. The Inferi are still seeking out Voldemort, and they can sense the horcruxes, seeking them out, including Harry. Voldemort goes ballistic when he discovers that the cave is compromised and the Locket missing. The Muggles start dropping hydrogen bombs all across the south of England. The Inferi are somehow imbibed with Hermione's intellect, making them cunning and dangerous. Hermione goes insane with the guilt of millions of innocent lives on her hands.