Last month, China was struck with its worst outbreak of Covid-19 in months—183 infections across Xinjiang, a city in the western province of Kashgar. While the majority of cases traced back to a garment factory, we now know the true source: contaminated freight trucks.

China is proving to be our canary in the coal mine when it comes to rare—yet consequential, as the Xinjiang outbreak shows—types of Covid-19 transmission. Two I’ve written about previously, cold supply chains and longer incubation periods. Contaminated freight makes a third. All three contradict guidance issued by the U.S. Centers for Disease Control and Prevention (CDC). All three are also largely undetectable in countries where disease surveillance is crippled by a constant influx of new cases.

Patient zero of the Xinjiang outbreak is suspected to be a male loading worker who, at the time of contagion, was handling trucks of export goods, according to the China Center for Disease Control and Prevention. Genomic sequencing and analysis later uncovered traces of virus on the interior surfaces of multiple trucks. The agency’s chief epidemiologist, Wu Zunyou, noted that it is unclear whether the man contracted the virus from a contaminated surface or contaminated air. Either way, the evidence urges us to recognize a hazard heretofore largely dismissed—domestic and in particular international freight.

Those of us currently in the thick of crisis have reason to pay attention to how China attends to its outbreaks’ more unusual suspects. First, it only takes one person to start—or in China’s case, given their near total elimination of the disease, restart—an epidemic, and as such even the most uncommon phenomena warrant scrutiny and caution. Second, China has gone to extreme lengths to contain Covid-19, as have its citizens. What occurs sporadically in China will likely be much more prevalent in countries where the government and the people have a more relaxed approach towards containment, the United States among them.

Since viruses fare far better at the lower temperatures winter brings than warmer climes, containers left cold and wet will naturally aid and abet their survival. While the shipments in the Kashgar-bound trucks were neither frozen nor refrigerated, imports of frozen food have been linked to clusters of new Covid-19 cases along the eastern coast of China and, a hop and skip away, New Zealand. Chinese health authorities located virus on the packaging of frozen shrimp from Ecuador, squid from Russia, and chicken wings from Brazil, just to name a few. As recently as last week a handful of Covid-19 cases in Shanghai were linked to an air cargo container that held, once again, cold and frozen foods.

While the U.S. CDC guidelines acknowledge that Covid-19 can spread from the surfaces of objects, they also say it is “unlikely to be spread from domestic or international mail, products or packaging.” Now we know this might not be the case. The same is true of another form of transmission that has provoked policy change in China, but virtually nowhere else—infection by a small fraction of people who take longer than two weeks to get sick after being exposed to Covid-19.

The Chinese government has determined, according to local authorities in Shanghai, that as many as five percent of Covid-19 cases could fall into this category. Though lacking a specific citation, the claim isn’t without precedent. A study conducted by Chinese researchers early on in the pandemic and later published in New England Journal of Medicine pinpointed the incubation period between 0 to 24 days. For one man, government officials in Hubei Province reported in February, incubation took nearly four weeks. A preprint that analyzed more than 60 articles on the subject found the upper limit in some reports to be even higher—a maximum 34 days.

A figure like five percent may seem too slight to be significant, but we must remember it doesn’t account for the many others those five percent could infect if they emerge from a two-week quarantine based on the false assumption they’re in the clear. In the United States, daily case counts are now toppling 200,000. Five percent of that figure alone is 10,000—a drop in the bucket in the grand scheme of things, perhaps, but plenty enough to drive up the spread of disease.

Even contaminated freight, which so far has infected people by the tens and low hundreds, presents a risk that cannot be ignored, considering such outbreaks are bound to go forth and multiply into far greater proportions if not contained. (Not all countries are able to test 4.75 million residents in three days, as China did in the wake of the Xinjiang outbreak.) The practical consequence should be to sanitize our hands and surroundings far more frequently than we do now, especially after handling any packages or frozen foods delivered in these cold months. But more broadly speaking, we need to broaden the scope of our thinking around what causes and perpetuates the transmission of Covid-19 and revise our public health guidelines accordingly. Now that China has set an example, we won’t be able to claim we were caught unawares.