Antibiotics can cure colds and flu.

The word penicillin appeared in a paper by Alexander Fleming in the British Journal of Experimental Pathology in 1929, and for a decade it remained a laboratory curiosity, difficult to produce, unstable, of uncertain clinical utility. Then the war changed the logistics. Howard Florey and Ernst Chain at Oxford developed techniques for producing penicillin in sufficient quantities for clinical use, and the U.S. government coordinated industrial-scale production beginning in 1943. American troops in the Pacific and Europe received penicillin for bacterial wound infections, pneumonia, gonorrhea, and other conditions that had previously killed reliably. The results were dramatic.

By the time American civilians could obtain penicillin in 1945, it had acquired the aura of a medical miracle, and reasonably so. Bacterial pneumonia, scarlet fever, syphilis, streptococcal infections that had sent patients to early graves were suddenly treatable. The broad-spectrum antibiotics that followed through the late 1940s and 1950s extended the reach further. Chlortetracycline was approved in 1948, chloramphenicol in 1949, erythromycin in 1952. Each new agent treated a wider range of bacterial infections. The implicit conclusion that antibiotics could cure infection was correct, as far as it went.

It did not go far enough. The common cold is not caused by bacteria. It is caused by rhinoviruses, of which there are more than a hundred serotypes, and by a range of other viruses including coronaviruses and adenoviruses. Influenza is caused by influenza viruses. Antibiotics target bacteria, they disrupt bacterial cell wall synthesis, inhibit bacterial protein production, interfere with bacterial DNA replication. They have no mechanism of action against viruses, which do not have cell walls and which replicate through entirely different molecular machinery, hijacking the host cell's own apparatus.

The distinction was understood by bacteriologists from the beginning of the antibiotic era, but it did not immediately percolate into clinical practice or public understanding. Doctors in the late 1940s and 1950s, accustomed to the spectacular results of antibiotics against bacterial disease, sometimes prescribed them for viral infections on the grounds that preventing secondary bacterial infections was worthwhile, or simply because patients expected treatment and prescribing was easier than explaining. Patients, for their part, had learned that antibiotics made serious illness go away. When they came to the office with a cold or flu, they reasonably wanted the medicine that worked.

The Cochrane Collaboration, reviewing the evidence in the early 2000s, found no benefit from antibiotic treatment in uncomplicated upper respiratory viral infections across multiple randomized controlled trials involving thousands of patients. Antibiotic-treated patients recovered at the same rate as those who received no treatment. What the antibiotic-treated patients also received was exposure to the drugs' side effects, gastrointestinal disturbance, allergic reactions, as well as contribution to a larger epidemiological problem.

That problem was resistance. Each course of antibiotics, whether appropriate or not, applies selective pressure on the bacterial populations living in and around the patient. Bacteria with mutations conferring resistance to the drug survive; susceptible bacteria are killed. The resistant strains proliferate. Prescriptions for antibiotics to treat colds and flu, infections against which the drugs do nothing, contributed to the reservoir of antibiotic-resistant bacteria that by the end of the twentieth century had become one of the major challenges in infectious disease medicine. The drugs that had been produced as cures were undermined, in part, by being used against diseases they could not treat.

Public health campaigns throughout the 1980s and 1990s worked to shift both prescribing behavior and patient expectations. Progress was measured in decades, not years. By the 2000s, surveys showed that substantial proportions of adults in the United States and Europe still believed antibiotics were appropriate for colds and flu. The miracle had reshaped expectations in ways that the explanation of viral biology struggled to undo.