Five poultry workers at a farm in northeast Colorado have been infected by the H5N1 virus, according to Colorado’s department of public health.

The workers were culling birds at a commercial poultry farm that had been affected by the virus. Their symptoms were mild, and included common “flu-like” respiratory symptoms — such as fever, chills, coughing, sore throat and runny nose — and conjunctivitis, or pink eye. None of the workers were hospitalized.

The virus also has been identified in the state’s dairy cattle and wild birds.

The U.S. Centers for Disease Control and Prevention has confirmed four of the five cases.

Colorado’s health department said the fifth case was presumptive, and pending confirmation.

The CDC said its risk assessment for the general public remains low, and its recommendations have not changed.

“An assessment of these cases will help inform whether this situation warrants a change to the human health risk assessment,” said the agency in a statement released Monday.

These are the first cases of H5 virus infection in poultry workers since 2022. The first ever U.S. case of H5N1 in a poultry worker was reported in Colorado in April 2022.

There have also been four cases of H5 in dairy workers. One of those cases was located in Colorado.

The CDC has sent a bilingual field team into the area to support the state’s investigation. The team of nine people includes epidemiologists, veterinarians, clinicians and an industrial hygienist.

Public health officials say it is safe to eat properly handled and cooked poultry products.

Essexella fossils date back to the Carboniferous period, when northern parts of Illinois hovered just above the equator. A local river delta fed into the sea, creating a network of brackish wetlands home to sea scorpions, centipedes and early amphibians. Many of these creatures were buried by mudslides, which protected their remains from scavengers and decay. In the 19th century, coal miners began excavating an area, known as Mazon Creek, for fuel, and the fossils turned up in their spoil heaps.

Collectors have been finding the remains of these critters in the Mazon Creek fossil beds for more than a century. Most of the fossils are entombed in ironstone nodules. Cracking these concretions reveals the imprints of soft-bodied animals that resemble bulge-eyed aliens. In the 1950s, a local collector named Francis Tully discovered the imprint of a torpedo-shaped creature with a nozzlelike mouth. The taxonomic identity of the “Tully monster” has perplexed researchers ever since.

Essexella was similarly perplexing. Nondescript fossils turned up by the thousands at Mazon Creek, and they were often sold at local flea markets, or even discarded.

Scientists published the first detailed scientific description of the blobs in 1979. Essexella fossils are composed of two structures — a textured, barrel-shaped region and a smooth bulb. Researchers posited that the textured area represented a skirtlike curtain that wrapped around jellyfish tentacles. The rounded region was the jellyfish bell.

But as time passed, this description struck many researchers as odd.

“We were really shoehorning it to fit the jellyfish model,” Dr. Plotnick said.

No living jellyfish have curtains around their tentacles. Such a curtain would make swimming and feeding cumbersome. The uniform shape of the blob fossils also perplexed Dr. Plotnick. “If it was a jellyfish that fell on the seafloor, it would just splatter out in all directions like an old string mop on the floor,” he said.

Dr. Plotnick tested some other hypotheses to explain the blobs — such as gelatinous, barrel-shaped critters called salps or colonial congregations of tiny creatures known as siphonophores — but each new identity failed to explain Essexella’s anomalous anatomy.

In late 2016, Dr. Plotnick and a colleague, James Hagadorn, a geologist at the Denver Museum of Nature and Science, investigated the motherlode of blobs. They were at the Field Museum in Chicago, a repository for Mazon Creek fossils that has the world’s largest Essexella collection. Most had been donated by amateur collectors who were too intrigued to leave the fossils in the scrap heap.

The scientists sifted through drawer after drawer of the splotchy specimens. They lined up several fossils to photograph and compare side by side on a table. One of the blobs caught Dr. Plotnick’s eye. As he rotated the fossil upside down, he was struck by the clarity that the change of perspective offered.

“It looked like the bottom of an anemone,” Dr. Plotnick said. He added, “That was one of only a few times I’ve actually had the classic eureka moment.”

As Dr. Plotnick brushed up on sea anemone anatomy, the ambiguous blobs came into focus. “All the things that bothered us about this being a jellyfish now makes sense,” he said.

Instead of being a jellyfish’s bell, the rounded region of the Essexella was an anemone’s burrowing base. The textured barrel was not a tentacle-enclosing curtain but the body of the anemone. Some specimens are preserved so well that the scientists could see the muscles that the anemone used to bend and contract.

Dr. Plotnick, Dr. Hagadorn and their team redescribed Essexella as an ancient anemone last year in the journal Papers in Palaeontology. Because of their soft bodies, ancient anemone species are mostly known from only a handful of poorly preserved fossils. With thousands of relatively well-preserved Essexella specimens, this once puzzling species is now the best-known anemone in the fossil record. Dr. Plotnick posits that these animals once lined the floor of the Mazon Creek estuary.

This isn’t the only time that paleontologists have flipped the scientific script to clarify the identity of a bizarre fossil. Reconstructing any ancient animal is tricky. After millions of years in the ground, fossils have been warped and weathered, crushed and scattered and stamped flat onto slabs of stone.

Sometimes, a fossil’s preservation alone is enough to disorient researchers. For decades, paleontologists were stumped by why armor-clad dinosaurs called Ankylosaurs were almost always fossilized upside down. In 2018, a team posited that the heavily armored animals often went belly up because of bloating as their carcasses floated out to sea.

And then there are the evolutionary oddballs that are difficult to decipher no matter the orientation of their fossils. In 1869, the paleontologist Edward Drinker Cope mistakenly placed the skull of an Elasmosaurus, a marine reptile, at the end of the creature’s tail instead of its elongated neck. Othniel Charles Marsh, another paleontologist, seized on Cope’s error, igniting a rivalry that would fester into the so-called Bone War.

Even weirder was Hallucigenia. For decades, researchers could not make heads or tails of the creature, a worm covered in tentacles and stiltlike spines. Then they realized that its head was really its tail, and vice versa. “That was fun and not a mere detail,” said Jean-Bernard Caron, who is a paleontologist at the Royal Ontario Museum and a co-author of a study in 2015 that determined a bulb on one end of the Hallucigenia was the creature’s head. Better-preserved fossils of a related animal in China also revealed that Hallucigenia, like Essexella, was originally reconstructed upside down.

“Clearly Hallucigenia has seen many flips,” Dr. Caron said.

While Dr. Caron’s work helped straighten out Hallucigenia, a recent paper upends his 2012 description of Pikaia, an enigmatic wormlike creature from the Burgess Shale in Canada that was purported to be an early forerunner to vertebrates. The new study suggests that a mysterious tubelike organ that researchers thought ran along Pikaia’s back (and may have been an early nerve cord) is actually the animal’s gut cavity, running along its belly.

“The animal is now on its head!” Dr. Caron said. Yet another fossilized creature got a new story when it turned over.

The virus has — so far — caused only minimal illness among humans, yet has spread rapidly among birds, sea mammals and other species with devastating effect.

Although U.S. health officers have repeatedly assured Americans that H5N1 bird flu poses little risk to their well-being, some experts have become increasingly critical of what they see as the government’s failure to aggressively monitor the spread of virus among cattle and other farm animals. The virus has been reported in 145 dairy herds across 12 states, but critics say this is likely an underestimate.

They point to stories they have heard anecdotally from physicians and veterinarians in farming communities about mystery illnesses and cover-ups. And they point to perplexing “hits” of H5N1 in municipal wastewater far from any infected dairy herds.

“I think our government officials are are not doing the thorough investigation they should be doing,” said Rick Bright, a virologist and the former head of the U.S. Biomedical Advanced Research and Development Authority. “I think they are continually minimizing this outbreak and this virus.”

The worry among some experts is that H5N1-infected farm animals could serve as “mixing vessels” for new viral strains that could more easily infect people. They point to research released this week showing that the virus has receptors for both birds and humans. And they note that up to 75% of human infectious diseases are derived from pathogens that originated from animals.

Now, amid this heated debate over viral monitoring, a raw milk dairy farmer and longtime critic of the Food and Drug Administration has accused the government of ignoring his tip about a suspected H5N1 outbreak among a herd of dairy cows in early May.

Although it remains unclear whether an outbreak actually occurred, neither federal nor state officials investigated the matter for weeks. Only after inquiries by The Times did officials announce Tuesday that they would look into the report.

The delayed response, some experts say, suggests a disturbing lack of oversight on the part of government officials.

“I think that once everybody decided it wasn’t going to kill people and pasteurized milk and herds could get over it, then all that was left was to take these stopgap measures — voluntary reporting, voluntary testing, testing when going across state lines — that don’t look anything like a really serious effort to go ahead and stamp it out,” said Michael Payne, a researcher and outreach coordinator at the Western Institute for Food Safety and Security at UC Davis.

The outbreak claim comes from Mark McAfee, owner of Raw Farms, a raw milk dairy producer with herds in Fresno and Hanford. On June 17, McAfee — who is also the president of the Raw Milk Institute, an advocacy group — emailed the FDA’s acting director, Donald Prater, to say that he’d been told a raw milk dairy herd had been infected and that people had probably consumed the contaminated milk.

According to McAfee’s email, which was shared with The Times, a subset of a farmer’s cows were suffering yellowish, runny diarrhea; low milk production; thick, yellowish colostrum; and general weakness. The farmer told McAfee he had separated the sick cows — about 10% of his herd — and discontinued milking them.

Mark McAfee walks by cows feeding at his raw milk dairy.

(Tomas Ovalle/For The Times)

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“The farmer reported that for certain, humans had consumed the raw milk at some level,” McAfee wrote to Prater, adding that the farmer’s veterinarian “told the farmer to not report anything to anyone” because the virus would pass and “he did not want the FDA to swoop in and cause a media frenzy.”

McAfee said he fields lots of questions and calls from raw milk farmers around the nation. Because he’s president and founder of the Raw Milk Institute — and the largest producer of raw milk in the country — his advice and counsel is often sought by smaller dairy farmers.

Within 90 minutes of sending the email, Prater responded that he appreciated the time McAfee had taken to write the note and “for sharing these perspectives.” He then added that he and his agency would “take note of the points you raised and come back to you if we have any questions.”

According to McAfee, the FDA did not follow up with him. The state of New Mexico, where McAfee says the herd was infected, was made aware of the tip only last week, after the Times inquired.

The U.S. Department of Agriculture and the Centers for Disease Control and Prevention would not comment for the record on the matter, referring questions to the state.

Critics say whether the outbreak occurred or not, the lack of an immediate, or even timely, response underscores the absence of urgency and leadership in the face of a potential health threat.

“If you turn your back on this virus, you’re kind of inviting it in to bite you in the ass,” said Bill Hanage, associate professor of epidemiology and co-director of the Center for Communicable Disease Dynamics at Harvard University’s T.H. Chan School of Public Health.

Bright, the virologist, said the delay was a problem.

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1. Carlos Rodriguez squeezes a cows teat to check the quality of the milk coming out of it. 2. Alvaro Hernandez hangs a milking cluster high to avoid contamination after a cow has been milked. 3. Mark McAfee checks bacteria levels in his raw milk every day and spends over $300,000 on testing. (Tomas Ovalle/For The Times)

“This is the stuff that drives me crazy,” Bright said. “I always tell people, it’s not necessarily the data that we have in front of us that’s most concerning. It is what’s being hid from us that’s most concerning.”

Farmers do not have to let government officials test their cattle, he said, which makes it hard to verify weeks after the fact whether an outbreak had occurred. If a dairy farmer allowed it, researchers could check for H5N1 antibodies in the blood.

“I really don’t think they want to know,” he said of the U.S. government, and the FDA in particular.

To be sure, the FDA and McAfee’s Raw Milk Institute have had a long, contentious history.

“To be quite frank, your source, Raw Milk Institute, are known advocates for repeatedly amplifying what they say is a lack of evidence that drinking raw milk with H5N1 is bad for you,” said Janell Goodwin, an FDA spokeswoman.

Indeed, McAfee said he reported the outbreak because he believed it showed that unpasteurized, yet infected raw milk was not a threat.

“No one got sick from that outbreak,” he insisted.

Mark McAfee, a raw milk dairy farmer and longtime critic of the FDA, has accused officials of ignoring his tip about a possible H5N1 outbreak among dairy cows.

(Tomas Ovalle/For The Times)

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But epidemiologists The Times spoke with expressed skepticism on this point, suggesting that maybe nobody had “reported” being sick, noting that many dairy workers are migrants who may not relish a government visit. And they pointed to experimental and observational studies of barn cats that consumed H5N1 contaminated raw milk and “had about a 50% mortality rate and really unpleasant symptoms,” said Hanage, the Harvard professor.

Drinking raw milk is “something that is a risk that we would rather people didn’t take just for their own sake,” he said.

McAfee noted that he has received a certificate of good standing by California’s Department of Food and Agriculture for voluntarily testing his cows’ milk for H5N1. A spokesman for the agency confirmed that as of July 1, the farm’s milk was clear of the virus.