In Los Angeles in 1924, after a series of mysterious deaths, Yersinia pestis, or bubonic plague, was swiftly identified as the culprit. Immediate quarantine of exposed people in selected areas helped to make the outbreak less than a devastating epidemic. But some public officials and newspaper reporters, in a desperate attempt to explain the origins of the illness, began equating the disease with people of Mexican descent. Panic quickly struck the city. Hotels and restaurants fired thousands of Latino workers. Health officials destroyed houses in low-income neighborhoods deemed “public health menaces” without offering compensation to the displaced. Fear of the disease rapidly attached itself to deep-seated fears of low-income individuals and foreigners and produced an unacceptable set of practices in the name of preventing an epidemic.

But is fear always detrimental in the face of potential epidemics? In other words, is fear ever, in fact, a useful preventative tool? Excessive, widespread panic may, of course, be counterproductive in the fight against emerging microbial threats. But small doses of fear, instilled carefully and systematically by epidemiologists and public health officials, may be vital in certain situations.

Complacency and lack of fear might have a role to play in the recent dramatic drops in vaccination rates in the U.S. and other developed countries. In states in which philosophical exemptions to vaccines are permitted, the rate of this voluntary opting out was 2.6% in 2009, up from 1% in 1991. A certain misplaced form of fear might in fact be driving this phenomenon in some cases: parents have latched onto the false belief that vaccines cause autism and overestimated risks associated with vaccination. But recent declines in vaccination rates may also be due to a lack of appropriate fear. Sixty years ago, the consequences of life without most modern vaccines were eminently visible: polio paralyzed 16,000 Americans every year; rubella caused birth defects in 20,000 babies; measles infected 4 million children and killed 3,000 annually. Today’s parents view infant mortality as a phenomenon of the past or a devastating problem relegated to developing countries. Parents in the second decade of the 21^st century have probably never witnessed an infant suffering from pertussis or diphtheria. Newly trained pediatricians may not know what a Koplik spot is, let alone have seen one The threat of child mortality is not looming in developed countries, and many parents do not realize that, in the absence of vaccines and proper herd immunity, these illnesses, as well as infant death, could once again become immediate threats. In the case of declining rates of infant vaccination, might not a small dose of fear help?  If parents were more frightened about the consequences of not vaccinating their children, whooping cough epidemics such as the one in Washington State this past year might be prevented.

When is it appropriate to sound the alarm bell and when should public health officials keep the public calm? Obviously, hysteria is never a useful response to any looming epidemic or public health threat. But in the case of emerging and not immediately apparent threats, such as the possibility of a pertussis outbreak or the growing rate of antibiotic resistance, it might be necessary to jolt people out of their complacency. Antimicrobial resistance is a complex problem, but there are a number of ways that we can reduce the damage. In many countries, there is no national regulation on antibiotics, healthy animals are given antibiotics for growth or to prevent disease, antibiotics can be purchased over the counter, and doctors prescribe antibiotics too readily. In this case, sounding the alarm bell loudly and repeatedly is absolutely necessary. On the other hand, in a case such as the 2003 outbreak of SARS, the amount of fear may have been out of proportion with the threat at hand, may have produced destructive mass hysteria, and may have resulted in a perhaps offensive avoidance of Asians and Asian countries.

Especially in cases of unknown, emerging microbial threats, health officials must help the public walk the line between healthy fear and detrimental widespread hysteria. Heightened fear of threats that are looming but not immediately in sight may sometimes be justified as a stimulus to preventative action. At the same time, fear of disease must be appropriately contained, in proportion with the crisis at hand, and must never become a proxy for irrational fears, such as xenophobia, that are all too easily stimulated in the face of complex health emergencies.