After bird flu hit the nation’s dairy farms, it spread with alarming speed. Since March, the virus has infected more than 850 herds in 16 states. It has also infected at least 60 people, nearly all of them farmworkers. In recent months, cases in cows and humans have mounted especially rapidly in California, now the epicenter of the outbreak.

Initially, experts worried that the virus might be spreading through tiny, airborne droplets that cows exhaled.

But data strongly suggest that the virus, known as H5N1, has spread primarily through milk. It replicates quickly in the udders of infected cows, which produce milk with sky-high levels of the pathogen. Droplets of milk can splash into dairy workers’ faces, while milk-splattered equipment and vehicles can transport the virus from cow to cow.

Although pasteurization effectively inactivates the virus, the pathogen was recently detected in retail samples of raw milk in California. The U.S. Department of Agriculture announced this month that it would begin testing the nation’s milk supply to help curb the outbreak.

Experts are nervous about the growing public health threat — and the potential ascension of Robert F. Kennedy Jr. to lead the Department of Health and Human Services. Kennedy is a vocal proponent and self-professed consumer of raw milk and has said that he wants federal researchers to take a “break” from studying infectious disease.

In theory, a virus that spreads through milk should be easier to control than one that floats invisibly in the air. But a look inside the modern dairy industry reveals that milk-based transmission is profoundly worrying. “Milk is hugely problematic,” said Seema Lakdawala, a virus researcher at Emory University.

The United States has more than 9 million dairy cows, which produce some 600 million pounds of milk per day. Over the past few decades, consolidation has transformed the industry; the number of dairy farms has plummeted, but their size has ballooned. In 1997, fewer than 18% of the nation’s dairy cows lived on farms with 1,000 or more animals; by 2022, 65% of them did.

These vast, messy milking operations are designed to extract as much milk as possible from each cow two or three times a day. A virus that gets into the milk of a single cow can quickly find its way to many others — or into the dairy industry’s largely immigrant workforce.

As the risks compound, here’s a look at how milking gets done on one big dairy farm. The farm has not had any known cases of bird flu, but the process reveals why a virus that spreads through milk is so hard to stop.

Hands-On Workers

A worker treats a cow with iodine, a disinfectant, at the Heeg Brothers Dairy farm near Colby, Wis. (Tim Gruber/The New York Times)

Heeg Brothers Dairy, in Colby, Wisconsin, is home to about 1,730 cows, said Jay Heeg, a third-generation dairy farmer. The primary milking parlor has 28 milking stalls in two parallel rows. It serves 1,050 cows, each of which is milked three times a day.

Farmworkers begin by manually squeezing a small amount of milk out of each teat. That step stimulates the flow of milk, helps flush out bacteria and allows the workers to examine the milk for any potential abnormalities.

Then, the workers dip each teat in iodine, a disinfectant. This precaution long predates the arrival of bird flu; it helps prevent any bacteria that may be on a cow’s udder from spreading into the milk supply or to other cows that will be milked by the same machine.

Then the workers dry the teats and attach a milking claw, a cluster of four stainless steel, rubber-lined teat cups, which draw out the milk.

Milk meters monitor the flow of the milk, which travels through hoses and pipelines to the milk house, where the liquid is rapidly cooled and pumped directly into delivery trucks.

When the flow of milk slows, the claw automatically detaches, and the teat cups swing through the air, sometimes spraying droplets of milk. On a farm with infected cows, dairy workers, who are usually positioned several feet lower than the animals, may be infected if virus-laden milk enters their eyes. The virus could also spread — to human workers or other cows — through tiny, airborne droplets of milk.

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The claws are not cleaned between cows, and lingering milk droplets may remain in the teat cups when they are attached to the animals next in line. If there is any virus in the milk, it could remain infectious inside the rubber liner for several hours, Lakdawala and her colleagues found.

That means that when the milking machine is attached to the next cow in line, the virus could travel into that cow’s teats. “During milking, the teat end is open, potentially even relaxed,” said Dr. John Barlow, a veterinary epidemiologist at the University of Vermont.

It takes seven hours for the farm to milk all 1,050 cows. Then, the workers spend an hour cleaning, which includes washing out the milking equipment with detergent and hot water. After that, the next eight-hour shift begins, and all the cows are milked again.

The Robotic Approach

A cow after being milked by the robotic milker at the Heeg Brothers Dairy farm near Colby, Wis. (Tim Gruber/The New York Times)

Some types of milking parlors could lower risks. These include robotic parlors, where the cows have been trained to approach automated milking machines on their own. The technology has been slow to take off in the United States, but it is increasingly popular.

Last December, Heeg Brothers Dairy opened its own robotic milking barn, which is home to about 450 cows and eight robots. To reach their feed, the cows pass through an automated “sort gate,” which scans a radio frequency identification, or RFID, tag implanted in each animal’s ear. If a cow has recently been milked, the gate opens into the feeding area. But if she’s due for milking, the gate diverts the cow into what is known as the commitment pen.

The barn has two of these pens, each outfitted with four robotic milking stations. Cows can linger in the pen as long as they want, but when they approach a station, a large robotic arm, equipped with a camera and sensors, performs the same tasks that human workers would otherwise do manually: squeezing a little milk from each teat, disinfecting the teats and attaching the claw. When milking is complete, the teat cups detach and the cow plods on its way.

For farmers, the big advantage of robotic systems is economic, reducing the need for human workers. Heeg said that he was also drawn to the robotic approach because it seemed less stressful for the cows, who are milked more on their own rhythms. “They just all get to do their own thing,” Heeg said.

Robotic parlors also mean that human workers do not need to stand next to cows’ udders all day, reducing opportunities for cow-to-human transmission of bird flu.

Moreover, these robots are programmed to rinse themselves with a mild iodine solution between every cow, which should reduce the spread of pathogens. (Some, though not all, conventional milking parlors have this same type of “backflush” system, Heeg said.) And because different groups of cows have their own dedicated robots, there are also fewer cows sharing the same milking equipment.

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Robotic parlors are not risk-free, of course; automated systems can break down and people are still needed for an assortment of cleaning and caregiving tasks. And in both kinds of parlors, milk can find its way into the air and the environment.

So far, the virus has not been confirmed on any dairy farms in Wisconsin, and dairy farmers face a lot of other stressors, including extreme weather and unpredictable prices, Heeg said.

To many farmers, he added, bird flu is just another in the long list of things that could make it harder to earn a living. “I’m not scared of it, but I don’t want it,” Heeg said. Still, he acknowledged, “I think it will be coming.”

This article originally appeared in The New York Times.