The U.S. Centers for Disease Control and Prevention (CDC), an agency that is currently focusing on the core mission reflected in its name, until recently was darkly warning us about a very different kind of "epidemic": an increase in e-cigarette use by teenagers, coupled with an outbreak of vaping-related lung injuries. The first concern did not involve any sort of disease; the latter did, but unlike COVID-19, the condition that the CDC dubbed "e-cigarette, or vaping, product use-associated lung injury" (EVALI) was not a contagious illness caused by a microorganism. And contrary to the CDC's misleading nomenclature and dangerously misguided initial advice, the two developments appear to be completely unrelated.

The CDC's switch from vaping to COVID-19 highlights the moral and practical differences between actual epidemics and metaphorical epidemics of risky behavior, both of which fall under the all-encompassing umbrella of "public health." Even within that framework, which is built around minimizing morbidity and mortality, the CDC's conflation of EVALI with vaping in general was counterproductive, impeding the harm-reducing shift from conventional cigarettes to nicotine delivery systems that are far less dangerous. By fostering confusion about the relative hazards of smoking and vaping, the CDC damaged its credibility on the eve of a public health crisis in which policy makers and the rest of us are expected to rely on its expertise.

Vaping is something people choose to do. In that respect it resembles many other phenomena that activists, politicians, bureaucrats, and academics have described as epidemics or public health problems, including smoking, overeating, using illegal drugs, riding a motorcycle or bicycle without a helmet, gambling, playing violent video games, and watching violent movies or pornography. COVID-19, by contrast, is something that happens to people. They do not consciously choose to be infected, although they can do things that increase or decrease that risk.

While there is plenty of room for debate about the legality, propriety, and effectiveness of specific policies aimed at curtailing the spread of COVID-19, there is a stronger argument for coercive measures when we are confronted by a potentially deadly disease that moves from person to person. That argument is much less compelling when we are talking about self-regarding actions that may lead to disease or injury but do not inherently endanger other people.

The habit of describing nearly anything that large numbers of people do as an epidemic, depending on whether it might compromise their health or whether others view it as pernicious, elides this crucial distinction. If protecting public health is presumed to be a legitimate function of government, an open-ended definition of that term is a prescription for constant political meddling in personal choices through taxes, regulations, and prohibitions as well as state-sponsored propaganda. Likening choices to contagious diseases invites the government to act as if those choices, and the personal tastes and preferences underlying them, morally matter as much as a virus's evolutionary imperative to survive and reproduce by infecting human hosts.

Once a particular pattern of behavior has been defined as an epidemic, that framing can lead to policies that make no sense even if you accept the collectivist calculus at the heart of the "public health" mission. That is what happened with drug prohibition, which is ostensibly aimed at reducing the harm associated with drug use but in practice increases the hazards faced by people who defy it, exposing them not only to the risk of arrest but also to black-market violence, potentially deadly variability in quality and potency, and communicable diseases fostered by a legal environment in which sanitary injection equipment is hard to obtain and risky to possess.

That is also what happened with vaping, which the CDC was predisposed to view as problematic, a prejudice that colored its depiction of EVALI. Even though it was clear early on that vaping-related lung injuries overwhelmingly involved black-market cannabis products, the CDC repeatedly intimated that legal, nicotine-delivering e-cigarettes might kill you. That message endangered public health by implying that people—teenagers as well as adults—would be better off smoking, which is demonstrably not true.

Only belatedly did the CDC recalibrate its guidance to focus on the potential hazards of "THC-containing e-cigarette, or vaping, products, particularly from informal sources like friends, family, or in-person or online dealers." It also foregrounded a warning that "adults using nicotine-containing e-cigarette, or vaping, products as an alternative to cigarettes should not go back to smoking"—advice that surely is equally sound for teenagers who are vaping rather than smoking.

We have to hope that the CDC's COVID-19 recommendations are more scientifically grounded than its initial scaremongering about e-cigarettes. I assume they are. But when public health agencies stray beyond their central mission and allow moral panics to affect their advice, they undermine the public trust that is vital at a time like this.