Just spread an infectious agent, tell people that established medical treatments or vaccines for it are peddled by liberals, tell people that known prevention methods are a liberal conspiracy to destroy America, tell people that social contact is their right and an act of resistance to the God-hating liberals, and encourage them to use a known toxic substance as "treatment." You can likely spread the infectious agent by finding a ultra-right cult conspiracy leader to promote a harmful product you're selling and then just infecting the product. You'll need to think about the agent spreading beyond your target, so you'll likely want to go with an agent that has a vaccine and established treatment but doesn't mutate much. This will help protect you while still infecting your target, who does not believe in established medical treatments or prevention methods. Feel free to go with the most fatal infectious agent meeting these criteria that you can find: in fact, the more fatal your agent is, the more likely it is that people in the US will resist established prevention and treatment, while insisting on their right to spread it.