A man goes to a birthday party, sits next to someone with hantavirus, catches it, gives it to his wife, and dies. His wife then infects 10 more people at his wake. Another guest at that same birthday party has no interaction with the index patient except to say “hello” as they cross paths, but that person gets sick too.One index patient, 33 subsequent infections, 11 deaths, four waves of transmission.This is from a meticulously documented hantavirus outbreak in Argentina in late 2018 and early 2019, published in the New England Journal of Medicine (NEJM). Nearly the exact same Andes strain of hantavirus caused the recent outbreak on the Dutch cruise ship MV Hondius. Yet from the moment this latest outbreak hit the news last month, public-health officials have been claiming that this virus is spread through “prolonged close contact.” The evidence is not nearly so reassuring.In any outbreak, the single most important question is: How does it spread? The answer informs the guidance for everything else, including how to stay safe, which protective measures to put in place, and who should be notified during contact tracing. Get it wrong and everything else breaks down.[Read: ‘This is not going to be the next COVID’]We made this mistake at the start of the coronavirus pandemic, and the cost was high. Health officials thought the virus spread on surfaces (“fomite transmission”) and through large droplets that dissipate quickly and can’t travel six feet. That’s why we spent a full year cleaning elevator buttons and putting stickers on floors telling people where to stand. But these interventions did little to halt the spread of a disease that in fact traveled through small particles that lingered dangerously in poorly ventilated and enclosed spaces.We’re now getting it wrong again. “This is not a respiratory disease,” Mike Waltz, the U.S. Ambassador to the United Nations, said about the hantavirus in an ABC News interview on Sunday, adding, “It’s very rare to see it transmitted between humans.” Transmission of the virus “requires close contact,” Jay Bhattacharya, the acting director of the Centers for Disease Control and Prevention, insisted last week. The CDC’s official communications have continued to emphasize that “prolonged, close contact” is necessary for transmission, as have other public-health officials outside the Trump administration.As an expert in what we call “exposure science,” I have spent a career conducting forensic investigations to understand how diseases spread and what we should do about it. As a member of the Lancet COVID-19 Commission, I chaired the Safe Work, Safe School, and Safe Travel task force, and was an early proponent of the theory that COVID spreads through the air. There was evidence early on of airborne transmission, which my colleagues and I tried to draw attention to. We modeled the early-2020 outbreak of the disease on the Diamond Princess cruise ship and found that 90 percent of the spread was through aerosols, not contaminated surfaces, but the CDC didn’t update its guidance until late 2020. I am alarmed to see the same pattern playing out now.Hantaviruses usually originate in rodent feces. Someone cleans a dusty area that has rodent droppings, inhales the particles, and gets sick. Only the Andes strain of hantavirus is known to be transmitted from human to human. In the outbreak documented in NEJM, the virus spreads without physical contact or prolonged exposure. One patient gets sick after simply crossing paths with someone who was ill. Two others are infected while seated at tables meters away. One person infected five others within 90 minutes at one party. The NEJM authors suggested that the virus spreads through the air.Although the NEJM evidence is clear, officials have kept repeating “prolonged, close contact,” so I wanted to be sure I wasn’t missing anything. Last week I spoke with a physician who was on the MV Hondius as a passenger but who jumped in to help treat infected passengers after the ship’s official doctor got sick and was evacuated. He told me that the original treating doctor and staff were definitely in close contact with the first patient. But the others who got sick? They had merely shared space in the dining room and the lecture hall, and had not had close contact. We’re now at 10 confirmed cases from the ship, which aligns with the prior outbreak dynamics: one person infecting many, no close contact required.Every outbreak investigation involves careful clinical workups, painstaking epidemiology, re-created time-activity patterns, and genomic sequencing—but almost every time, without fail, the investigators ignore the actual space where the outbreak took place. Was the cruise ship’s ventilation system working? What filters did it have, and were they running?[Read: What happened on the hantavirus cruise, according to a doctor on board]This matters because medical teams treating patients need to know how they might be exposed. When infected passengers go home to quarantine, their households need to understand the risk. As passengers fly back to their home countries, contact tracers need to know which exposures matter. The doctor who treated patients on the cruise said on CNN that he relied on goggles, a gown, and hand-washing to protect himself. But given that this virus spreads through the air, an N95 mask and a strong ventilation and filtration system would have served him better.This outbreak is not likely to spark a pandemic, mostly because the hantavirus is less contagious than influenza, measles, and SARS-CoV-2. But given just how little experience we have with this virus, any certainty is hubris. Thankfully, despite the flawed messaging, the system is broadly working: Officials are investigating, passengers are quarantined, the seriously ill are getting treatment, and the risk to the general public is low. International and national public-health authorities are acting responsibly.But what happens next depends on how well public-health officials communicate what precautions people should be taking. If people mistakenly believe transmission relies only on “prolonged close contact,” they may take risks they will soon regret.Public-health officials have to be more honest and more humble about how this virus actually spreads. An essential lesson from COVID is that officials should be candid about communicating that we are often learning in real time, and we should shy away from making bold pronouncements that may prove dangerously misleading weeks or months later. When it comes to preventing an outbreak from becoming a pandemic, insisting on the wrong answer to that most central question—How does it spread?—may well be worse than not having an answer at all.