Scientific denialism comes in different shapes and flavors, though not all are equally harmful: flat-Earthism, on its own, doesn’t seem to pose major risks; climate change denial, on the other hand, carries far heavier consequences. The picture becomes far more chilling when we face forms of opposition to scientific consensus that not only attack treatment and prevention campaigns against certain diseases (such as the anti-vaccine movement) but go so far as to question the very existence of the agents that cause them. We will review three decades of denialism in this field, from South African president Mbeki’s catastrophic &`doubts&´ about the viral origin of AIDS to claims that the COVID virus &`was never isolated&´ and therefore could not possibly be the target of immunization strategies. But since our aim is not only to describe these forms of denialism but to counter them before their unhealthy carriers take us down with them, we will explore two theories that from cognitive psychology and from the &`science of science communication&´ promise us some answers. Can we rationally persuade denialists? Maybe… or maybe all that’s left is to manipulate them.