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At first glance, some of the expert reactions to the recent surge in bird flu virus cases, both in the U.S. and around the world, may appear contradictory. Isn’t a more urgent response required? How much livestock will be sacrificed? Is the risk to humans really so low that only moderate actions are called for?

In truth, though, most of the basics are no longer in question among epidemiologists. This H5N1 virus is certainly spreading. Thousands of outbreaks have been documented in wild and farmed bird populations across all continents, spilling over into mammal populations. In the U.S. alone, bird flu has resulted in the death of more than 96 million birds in commercial and backyard flocks since February 2022, according to a USDA database.

‘The virus has proven its versatility’

Since 1997, sporadic H5N1 infections have been reported in humans in 24 countries, though relatively few cases were reported in recent years. After only one case in the U.S. in the previous 25 years, three farmworkers here have become infected over the past two months.

So where do we go from here? That depends significantly, the experts say, on whether governments and those involved are willing to conduct enough testing and surveillance to know where things really stand, and whether the results of that testing will be timely and transparent.

“I would like to see very widespread serologic testing done in humans—the farm workers, their family members, contacts,” says epidemiologist Michael Osterholm, director of the Center for Infectious Disease Research and Policy. (Serologic testing looks for antibodies in the blood.) “That way, we can see if we’ve had more transmission in humans that we’ve missed. We don’t have that right now.”

“There are just so many things we don’t know, and it’s the unknowns that concern us more than what we know so far,” says Rick Bright, a virologist, pandemic expert, and former head of the U.S. Biomedical Advanced Research and Development Authority.

Without question, researchers say, the H5N1 virus, also known as bird flu or avian influenza, is surging among mammals and proving itself very versatile at jumping from species to species. That includes the recent surprising spread to dairy cattle in the U.S. since March, the first such instance on record.

The virus has now been confirmed in cows in more than 85 herds with as many as 12 states affected. This geographic spread, along with high levels of exposure by workers at farms, slaughterhouses, milk processing facilities, and milk itself, is part of what has experts concerned that the virus will be found in more people. The Centers for Disease Control (CDC) has warned against drinking unpasteurized raw milk, which the CDC says may contain the H5N1 virus.

“The virus has proven its versatility to infect about any mammal it comes in contact with,” says Bright.

A recent H5N1 outbreak at a commercial egg farm in Iowa led to 4.2 million chickens being destroyed in order to prevent further spread. Multiple types of mammals have been infected, including domestic cats and dogs, red foxes, raccoons, bears, bobcats, and alpacas. Meanwhile, the presence of the H5N1 virus in house mice brings the virus literally closer to home, as rodents can be effective carriers.

“When you have a bird flu virus infecting mammal species, that raises the question of the virus becoming more adaptive for mammalian transmission,” says Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. “There are distinct barriers that bird flu viruses face when they’re infecting mammals in terms of which receptors they use. So that has raised the stakes.”

Whether that spread ultimately involves humans on a large scale is one of the unknowns. Among the three people who have been diagnosed in the U.S. since April, the most recent, a farm worker in Michigan, was the first from the current outbreak to exhibit mild upper respiratory symptoms.

The respiratory piece concerns health experts because someone with H5N1 virus in their airways could be more likely to spread the virus, perhaps via cough. To date, there’s been no evidence of human-to-human transmission of H5N1 virus, and the CDC says the current health risk to the general public is low. But as CDC Principal Deputy Director Nirav Shah noted at a Council on Foreign Relations event in May, “The risk here of something going from one or two sporadic [human] cases to becoming something of international concern (is) not insignificant.”

Though infections in the three individuals with the current strain of H5N1 virus have been mild, the overall death rate in humans from H5N1 infections since it was first identified in the late 1990s is more than 50% among the more than 900 confirmed cases worldwide. And this H5N1 strain has caused some alarming effects in animals.

Cats on a Texas farm died after drinking raw milk from bird-flu-infected cows, and test results showed “high amounts of virus” present in two felines’ brains and lungs who were tested. “It’s interesting that we’ve seen a lot of multi-organ involvement, brain involvement, major organ involvement that has been really remarkable causing the death of many of these different animal species. And we don’t understand that yet,” says Osterholm.

Reuters recently reported that dairy cows in five states have died or been slaughtered because they did not recover from their infections, though the USDA says the majority of cows do recover. Should infection among cattle become more widespread, the potential cost to American farms is astronomical, as cows cost much more than chickens or turkeys to raise—ironically, one reason farmers may be reluctant to test them.

“What we’re seeing right now in dairy cattle is just another situation where the potential for that virus to change is, I think, surely increased,” Osterholm says. The rise in severe illness among many of the species who’ve been infected by H5N1 is concerning, he notes, partly because it isn’t clear exactly why that has happened. “If you look at genetic sequences and look what the virus has done, we can’t explain that any one mutation has caused this to happen.”

‘We’re just sort of letting it go and spreading it in a very naive way’

The CDC recently confirmed the lethality of the H5N1 virus, isolated from a human infection in Texas, in ferrets that were experimentally infected with the same virus. All of those ferrets died, and Bright says scientists “found evidence that the virus had infected multiple internal organs such as the heart, lungs, and kidneys, and was also found in the brain and the blood.” The CDC said the findings underscore the potential for serious illness in people.

Osterholm and others are advocating for increased urgency on the testing front. This is a multifaceted ask; it includes more widespread and continuous testing of farm animals, workers, and their close contacts—and in the case of dairy herds, ongoing testing of the milk supply and any meat that is directed to the food supply for human or animal consumption.

It’s important to also test the farm environment, including equipment, transport vehicles, milking machines, and water systems that may be contaminated by disposal of infected milk. Raw milk from cows infected with bird flu has been found to contain astounding amounts of viral particles, according to a non-peer-reviewed study. (The FDA says our commercial supply of pasteurized milk remains safe to drink.)

Serology testing, which Bright says has been essentially nonexistent, could help researchers better understand the true extent of H5N1’s spread, which in turn would enable more effective containment strategies. But in the U.S., where there is no single controlling agency or government body coordinating the effort, the result has been patchwork and uneven.

“We’re not stopping it,” says Bright. “We’re not doing anything to keep that infected milk and infectious milk on the farm. We’re not testing these cows before they’re put back onto the milking line or sent to slaughter…We’re just sort of letting it go and spreading it in a very naive way.”

At almost every turn, there are complications. Absent a federal mandate, most of the testing being done is voluntary. A USDA spokesman told me the agency provides voluntary testing and monitoring options, and has a program that reimburses dairy producers for collecting samples. But, Bright says, “we are finding that many farms don’t want to test because they are afraid that they’ll be shut down or suffer a significant economic loss that is not yet being compensated by federal programs.”

As for the workers themselves, the CDC recommends that those in contact with infected or potentially infected animals, raw milk, etc. wear personal protective equipment (PPE), but only those exposed with flu-like symptoms should be tested (again, voluntarily). Further, most farmers want to work first with their county and state health agencies, preferring not to have federal health officials on their land. As of June 7, the CDC reported that only 45 people had been tested nationwide since March.

A proactive testing and surveillance program would make rapid flu testing available at all farms, Adalja says. (Specific H5N1 tests don’t yet exist.) “We would be randomly testing cows all over, not just ones that appeared sick or ones that are transported from one state to another,” he adds. And experts have suggested pooled testing of milk from more cows on each farm in an effort to detect infected cows that might not show outward signs of infection.

Sharing data quickly from cases of infected animals and people is critical, too, experts say. “The USDA is refusing to share the sequence data from the cows and the animals in a timely manner,” says Bright. “They have not shared a sequence that they collected from any infected animal in the last eight weeks.” And while the agency is sporadically submitting virus sequences to an international database, the data shared, Bright says, is largely from animals infected in March and early April.

Federal officials say they will have access to millions of doses of vaccine, should they be needed, as part of their strategic national stockpile. A company contracted by the U.S. government, CSL Seqirus, confirmed that it will fill roughly 4.8 million doses of pre-pandemic vaccine “that is well matched to the H5 of the currently circulating H5N1 strain.”

The federal, state, and local arms investigating and monitoring this strain of bird flu “have very collegial, collaborative, candid, discussions,” says Paul Friedrichs, the White House director of the Office of Pandemic Preparedness and Response Policy. “We don’t always agree, which is exactly what you expect from a complex situation, but what we always do is get to a decision and move out on it.”

For now, though, the researchers are consistent in their refrain about what’s needed: heightened surveillance and testing, testing, testing. “It is going to adapt,” Rick Bright says of H5N1. “We’ve watched it adapt over the years among bird species, and we know it’s what influenza viruses do.” The time to get ahead of that evolution, experts say, is now.

Carolyn Barber, M.D., is an internationally published science and medical writer and a 25-year emergency physician. She is the author of Runaway Medicine: What You Don’t Know May Kill You, and the cofounder of the California-based homeless work program Wheels of Change.

The opinions expressed in Fortune.com commentary pieces are solely the views of their authors and do not necessarily reflect the opinions and beliefs of Fortune.