Hanna D. Paton, University of Iowa The flu sickens millions of people in the U.S. every year, and the past year has been particularly tough. Although infections are trending downward, the Centers for Disease Control and Prevention has called the winter of 2024-2025 a “high severity” season with the highest hospitalization rate in 15 years. Since early 2024, a different kind of flu called bird flu, formally known as avian influenza, has been spreading in birds as well as in cattle. The current bird flu outbreak has infected 70 Americans and caused two deaths as of April 8, 2025. Public health and infectious disease experts say the risk to people is currently low, but they have expressed concern that this strain of the bird flu virus may mutate to spread between people. As a doctoral candidate in immunology, I study how pathogens that make us sick interact with our immune system. The viruses that cause seasonal flu and bird flu are distinct but still closely related. Understanding their similarities and differences can help people protect themselves and their loved ones.

What is influenza?

The flu has long been a threat to public health. The first recorded influenza pandemic occurred in 1518, but references to illnesses possibly caused by influenza stretch back as as early as 412 B.C., to a treatise called Of the Epidemics by the Greek physician Hippocrates. Today, the World Health Organization estimates that the flu infects 1 billion people every year. Of these, 3 million to 5 million infections cause severe illness, and hundreds of thousands are fatal. Influenza is part of a large family of viruses called orthomyxoviruses. This family contains several subtypes of influenza, referred to as A, B, C and D, which differ in their genetic makeup and in the types of infections they cause. Influenza A and B pose the largest threat to humans and can cause severe disease. Influenza C causes mild disease, and influenza D is not known to infect people. Since the turn of the 20th century, influenza A has caused four pandemics. Influenza B has never caused a pandemic.

An influenza A strain called H1N1 caused the famous 1918 Spanish flu pandemic, which killed about 50 million people worldwide. A related H1N1 virus was responsible for the most recent influenza A pandemic in 2009, commonly referred to as the swine flu pandemic. In that case, scientists believe multiple different types of influenza A virus mixed their genetic information to produce a new and especially virulent strain of the virus that infected more than 60 million people in the U.S. from April 12, 2009, to April 10, 2010, and caused huge losses to the agriculture and travel industries. Both swine and avian influenza are strains of influenza A. Just as swine flu strains tend to infect pigs, avian flu strains tend to infect birds. But the potential for influenza A viruses that typically infect animals to cause pandemics in humans like the swine flu pandemic is why experts are concerned about the current avian influenza outbreak.

Seasonal flu versus bird flu

Different strains of influenza A and influenza B emerge each year from about October to May as seasonal flu. The CDC collects and analyzes data from public health and clinical labs to determine which strains are circulating through the population and in what proportions. For example, recent data shows that H1N1 and H3N2, both influenza A viruses, were responsible for the vast majority of cases this season. Standard tests for influenza generally determine whether illness is caused by an A or B strain, but not which strain specifically. Officials at the Food and Drug Administration use this information to make strain recommendations for the following season’s influenza vaccine. Although the meeting at which FDA advisers were to decide the makeup of the 2026 flu vaccine was unexpectedly canceled in late February, the FDA still released its strain recommendations to manufacturers. The recommendations do not include H5N1, the influenza A strain that causes avian flu. The number of strains that can be added into seasonal influenza vaccines is limited. Because cases of people infected with H5N1 are minimal, population-level vaccination is not currently necessary. As such, seasonal flu vaccines are not designed to protect against avian influenza. No commercially available human vaccines currently exist for avian influenza viruses.

How do people get bird flu?

Although H5N1 mainly infects birds, it occasionally infects people, too. Human cases, first reported in 1997 in Hong Kong, have primarily occurred in poultry farm workers or others who have interacted closely with infected birds. Initially identified in China in 1996, the first major outbreak of H5 family avian flu occurred in North America in 2014-2015. This 2014 outbreak was caused by the H5N8 strain, a close relative of H5N1. The first H5N1 outbreak in North America began in 2021 when infected birds carried the virus across the ocean. It then ripped through poultry farms across the continent.

In March 2024, epidemiologists identified H5N1 infections in cows on dairy farms. This is the first time that bird flu was reported to infect cows. Then, on April 1, 2024, health officials in Texas reported the first case of a person catching bird flu from infected cattle. This was the first time transmission of bird flu between mammals was documented. As of March 21, 2025, there have been 988 human cases of H5N1 worldwide since 1997, about half of which resulted in death. The current outbreak in the U.S. accounts for 70 of those infections and one death. Importantly, there have been no reports of H5N1 spreading directly from one person to another. Since avian flu is an influenza A strain, it would show up as positive on a standard rapid flu test. However, there is no evidence so far that avian flu is significantly contributing to current influenza cases. Specific testing is required to confirm that a person has avian flu. This testing is not done unless there is reason to believe the person was exposed to sick birds or other sources of infection.

How might avian flu become more dangerous?

As viruses replicate within the cells of their host, their genetic information can get copied incorrectly. Some of these genetic mutations cause no immediate differences, while others alter some key viral characteristics. Influenza viruses mutate in a special way called reassortment, which occurs when multiple strains infect the same cell and trade pieces of their genome with one another, potentially creating new, unique strains. This process prolongs the time the virus can inhabit a host before an infection is cleared. Even a slight change in a strain of influenza can result in the immune system’s inability to recognize the virus. As a result, this process forces our immune systems to build new defenses instead of using immunity from previous infections. Reassortment can also change how harmful strains are to their host and can even enable a strain to infect a different species of host. For example, strains that typically infect pigs or birds may acquire the ability to infect people. Influenza A can infect many different types of animals, including cattle, birds, pigs and horses. This means there are many strains that can intermingle to create novel strains that people’s immune systems have not encountered before – and are therefore not primed to fight. It is possible for this type of transformation to also occur in H5N1. The CDC monitors which strains of flu are circulating in order prepare for that possibility. Additionally, the U.S. Department of Agriculture has a surveillance system for monitoring potential threats for spillover from birds and other animals, although this capacity may be at risk due to staff cuts in the department. These systems are critical to ensure that public health officials have the most up-to-date information on the threat that H5N1 poses to public health and can take action as early as possible when a threat is evident. Hanna D. Paton, PhD Candidate in Immunology, University of Iowa This article is republished from The Conversation under a Creative Commons license. Read the original article.

David Kitchen, University of Richmond Volcanoes inspire awe with spectacular eruptions and incandescent rivers of lava, but often their deadliest hazard is what quietly falls from the sky. When a large volcano erupts, as Mount Spurr appears close to doing about 80 miles from Anchorage, Alaska, it can release enormous volumes of ash. Fine ash can infiltrate the lungs of people and animals who breathe it in, poison crops and disrupt aquatic life. Thick deposits of ash can collapse roofs, cripple utilities and disrupt transport networks. Ash may lack the visual impact of flowing lava, but as a geologist who studies disasters, I’m aware that ash travels farther, lasts longer and leaves deep scars.

Volcanic ash: What it is, and why it matters

Volcanic ash forms when viscous magma – molten rock from deep beneath Earth’s surface – erupts, exploding into shards of rock, mineral and glass carried in a near-supersonic stream of hot gas. Towering clouds of ash rise several miles into the atmosphere, where the ash is captured by high-altitude winds that can carry it hundreds or even thousands of miles. As the volcanic ash settles back to Earth, it accumulates in layers that typically decrease in thickness with distance from the eruption source. Near the vent, the ash may be several feet deep, but communities farther away may see only a dusting.

Breathing danger: Health risks from ash

Breathing volcanic ash can irritate the throat and lungs, trigger asthma attacks and aggravate chronic respiratory conditions such as COPD. The finest particles pose the greatest risk because they can penetrate deep into the lungs and cause death by asphyxiation in the worst cases. Mild, short-term symptoms often resolve with rest. However, the long-term consequences of ash exposure can include silicosis, a lung disease and a possible cause of cancer. The danger increases in dry regions where fallen ash can be kicked up into the air again by wind or human activity.

Risks to pets and livestock

Humans aren’t the only ones at risk. Animals experience similar respiratory symptoms to humans. Domestic pets can develop respiratory distress, eye inflammation and paw irritation from exposure to ash.

Livestock face greater dangers. If grazing animals eat volcanic ash, it can damage their teeth, block their intestines and poison them. During the 2010 Eyjafjallajökull eruption in Iceland, farmers were advised to shelter sheep and cattle because the ash contained fluoride concentrations above the recognized safety threshold of 400 parts per million. Animals that remained exposed became sick and some died.

Harm to crops, soil and water

Soil and crops can also be damaged. Volcanic ash alters the acidity of soil and introduces harmful elements such as arsenic and sulfur into the environment. While the ash can add nutrients such as potassium and phosphorus that enhance fertility, the immediate impact is mostly harmful. Ash can smother crops, block sunlight and clog the tiny stomata, or pores, in leaves that allow plants to exchange gases with the atmosphere. It can also introduce toxins that render food unmarketable. Vegetables, fruit trees and vines are particularly vulnerable, but even sturdy cereals and grasses can die if ash remains on leaves or poisons emerging shoots. Following the 1991 Mount Pinatubo eruption, vast tracts of farmland in central Luzon in the Philippines were rendered unproductive for years due to acidic ash and buried topsoil. If multiple ashfalls occur in a growing season, crop failure becomes a near certainty. It was the cause of a historic famine that followed the eruption of Mount Tambora in 1815.

Ash can also contaminate surface water by introducing toxins and increasing the water’s acidity. The toxins can leach into groundwater, contaminating wells. Fine ash particles can also settle in waterways and smother aquatic plants and animals. During the 2008 Chaitén eruption in Chile, ash contamination led to widespread fish deaths in the Río Blanco.

Ash can ground airplanes, gum up infrastructure

Ash clouds are extremely dangerous to aircraft. The glassy ash particles melt when sucked into jet turbines, clog fuel systems and can stall engines in midair. In 1982, British Airways Flight 9 lost power in all four engines after flying through an ash cloud. A similar incident occurred in 1989 to KLM Flight 867 over Alaska. In 2010, Iceland’s Eyjafjallajökull eruption grounded more than 100,000 flights across Europe, disrupting travel for over 10 million passengers and costing the global economy billions of dollars. Volcanic ash can also wreak havoc on infrastructure by clogging water supplies, short-circuiting electrical systems and collapsing roofs under its weight. It can disrupt transportation, communication, rescue and power networks, as the 1991 eruption of Mount Pinatubo in the Philippines dramatically demonstrated.

What to do during ashfall

During an ashfall event, the most effective strategy to stay safe is to stay indoors as much as possible and avoid inhaling ash particles. Anyone who must go outside should wear a properly fitted N95 or P2 mask. Cloth masks provide little protection against fine ash. Rainwater tanks, troughs and open wells should be covered and monitored for contamination. Livestock should be moved to clean pastures or given uncontaminated fodder.

To reduce structural damage, ash should be cleared from roofs and gutters promptly, especially before rainfall. Older adults, children and people who are sick are at greatest risk, particularly those living in poorly ventilated homes. Rural communities that are dependent on agriculture and livestock are disproportionately affected by ashfall, as are low-income people who lack access to clean water, protective masks or safe shelter. Communities can stay informed about ash risks through official alerts, including those from the Volcanic Ash Advisory Centers, which monitor ash dispersion and issue timely warnings. The International Volcanic Health Hazard Network also offers guidelines on personal protection, emergency planning and ash cleanup.

The long tail of ash

Volcanic ash may fall quietly, but its effects are widespread, persistent and potentially deadly. It poses a chronic threat to health, agriculture, infrastructure and aquatic systems. Recognizing the risk is a crucial first step to protecting lives. Effective planning and public awareness can further help reduce the damage. David Kitchen, Associate Professor of Geology, University of Richmond This article is republished from The Conversation under a Creative Commons license. Read the original article.