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By Amanda Greenwood and Alice Gibbs

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The U.S. is tracking several serious infection threats this summer, ranging from foodborne parasites and bacteria to respiratory viruses and a drug-resistant fungus spreading in health care settings.

The outbreaks vary sharply in size, severity and how easily they spread.

Public health experts say assessing the significance of an outbreak involves more than simply counting cases.

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"Public health significance depends on the size of the outbreak, the severity of cases, how frequent the illness is, and how amenable the outbreak is to control by public health interventions," Paul Hunter, professor in medicine and specialist in infection and immunity at the University of East Anglia, told Newsweek.

"The outbreaks with the largest number of cases do not always cause the most concern."

Some, such as COVID-19 and measles, can spread directly between people. Others, including Cyclospora and Salmonella, are largely tied to contaminated food.

Candida auris—a fungal infection—is especially dangerous for already sick patients but is not considered a major risk to healthy people in everyday settings.

Based on the number of cases, geographic spread, risk of serious illness or death, and ease of transmission, here's how these infections rank:

1. Measles

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Measles ranks highest because it combines a large national footprint with extreme contagiousness.

U.S. measles cases have reached a 35-year high, with 2,318 confirmed cases reported nationwide this year as of late July, according to Centers for Disease Control figures.

Cases have been reported in 44 states and Washington, D.C., with many infections among unvaccinated people.

Hunter said measles is the outbreak that concerns him most as it "can cause substantial severe disease and death, and rarely death many years after the initial infection," he said.

"Measles can be controlled by vaccination, but uptake rates have been falling, and the risk for this disease is increasing on both sides of the Atlantic, so I fear even more infections in coming years unless we can reverse the decline in vaccine uptake."

Wendy Barclay, the regius professor of infectious disease at Imperial College in London, told Newsweek that measles was the outbreak that concerned her most among the infections currently circulating. "Measles is perfectly preventable by a very good vaccine which has been used for years, but what we see unfortunately is a trend away from having children vaccinated," she said.

Barclay described measles as "really, really highly contagious" and warned that declining vaccination rates could lead to larger outbreaks and severe complications.

"Measles doesn't only make a child feel very unhappy and very sick themselves, but there are some very nasty but rare sequelae of measles where it goes to the brain sometimes and that causes death," she said.

Measles is a viral illness that typically causes fever, cough, runny nose, red eyes and a rash. It can also cause pneumonia, brain swelling and death in severe cases.

Reuters reported that 151 measles-related hospitalizations had been recorded this year, while no deaths had been reported in 2026 at the time of the CDC update.

It’s ranked No. 1 because measles is one of the most contagious human diseases.

Unlike foodborne outbreaks, it can spread through the air when an infected person breathes, coughs, or sneezes, making it easier to catch than most other infections on this list.

2. COVID-19

COVID-19 ranks second because of its broad geographic spread and ability to transmit easily between people.

CDC modeling estimated that COVID-19 infections were growing or likely growing in 41 states as of July 29, 2026.

COVID-19 is caused by SARS-CoV-2 and can lead to fever, cough, sore throat, fatigue, congestion, shortness of breath, and, in some cases, severe respiratory illness.

CDC surveillance showed test positivity rising to 2.2 percent for the week ending July 18, up from 1.6 percent the previous week, while emergency department visits diagnosed as COVID-19 also increased to 0.2 percent.

COVID-19 is easier to catch than foodborne or vector-borne infections because it spreads through respiratory particles. However, current severity indicators remain relatively low compared with earlier pandemic waves, which places it below measles in this ranking.

Although COVID-19 continues to circulate widely, Hunter noted that the disease is generally causing less severe illness than during earlier stages of the pandemic.

"COVID is here forever and most of us can expect an infection every few years, but its severity has been in decline since the start of Omicron," he said.

Barclay said COVID-19 remains important because it continues to spread easily between people, though she stressed that current variants are generally causing less severe illness than earlier in the pandemic. "Most healthy people have either been vaccinated already or are infected and have immunity to it already and are not at huge risk," she said.

Barclay added that she was not surprised to see ongoing outbreaks because SARS‑CoV‑2 has become an endemic virus that is likely to continue causing seasonal waves.

3. Cyclospora

Cyclospora ranks third because of the scale of the outbreak.

As of August 5, CDC data showed 6,358 illnesses across 15 states, at least 278 hospitalizations, and two deaths in Michigan linked to a multistate cyclosporiasis outbreak tied to recalled iceberg lettuce from Taylor Farms de Mexico.

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Cyclospora is a microscopic parasite that causes cyclosporiasis, a diarrheal illness.

Symptoms can include:

• Watery diarrhea
• Appetite loss
• Stomach cramps
• Bloating
• Nausea
• Fatigue
• Low-grade fever

The infection is usually contracted by eating food or drinking water contaminated with feces.

Despite the large case count, Cyclospora isn’t ranked above measles or COVID-19 because it doesn’t spread easily from person to person. The major risk comes from contaminated produce, particularly raw fruits and vegetables.

Hunter said the outbreak is unusual because it appears linked to a specific source—the imported iceberg lettuce from Mexico.

"Except in people with severe immune suppression the disease is rarely fatal, though can last for several weeks," he said. "Person-to-person transmission is not that effective so I would expect the outbreak to peter out soon provided that the source of infection can be controlled."

4. Candida auris

Candida auris (C. auris) ranks fourth because it’s far more dangerous for vulnerable patients than most healthy people, but thankfully it doesn't spread easily in the community.

CDC data showed thousands of C. auris cases this year, with 3,437 cases in 27 states as of the week ending July 25.

C. auris is a drug-resistant yeast that can cause severe infections, including bloodstream infections.

It often spreads in hospitals, nursing homes, and long-term care facilities, especially among patients with weakened immune systems or invasive medical devices such as breathing tubes, feeding tubes, catheters, or central lines.

Barclay said the fungus presents a very different threat than respiratory or foodborne outbreaks because it primarily affects vulnerable patients: "For most healthy adults we might not be quite so worried about that, but if we've got to go into hospital ourselves, then that becomes worrying," she said.

Hunter reiterated this by saying C. auris poses its greatest threat inside health care facilities: "Candida auris is a hospital epidemic," he said. "It can become invasive, causing sepsis or meningitis." While potentially deadly for vulnerable patients, Hunter said it’s "not a big concern in the community."

Its seriousness comes from treatment resistance. CDC has previously estimated that 30 to 60 percent of people with C. auris infections have died, although many had other serious illnesses that also increased their risk of death.

5. Salmonella

Salmonella ranks fifth because the outbreak is widespread but, so far, less deadly than several others on the list.

CDC said 345 people across 27 states had been sickened in a Salmonella outbreak linked to recalled jalapeños from Sinaloa, Mexico, distributed by Coast Citrus Distributors. The CDC also reported that although 36 people had been hospitalized, no deaths had been reported.

Salmonella infection usually causes diarrhea, fever and stomach cramps, often beginning six hours to six days after exposure. Most people recover without treatment, but children under five, adults over 65 and people with weakened immune systems are more likely to become seriously ill.

"Salmonella is another foodborne infection and outbreaks occur most years," Hunter said. "Control again is by ensuring good food safety."

Salmonella is ranked below C. auris because it’s generally less deadly, and below Cyclospora because the current case total is much smaller.

6. Tularemia, or 'Rabbit Fever'

Tularemia ranks sixth because the current cluster appears small and localized.

Suffolk County, New York, reported at least three probable cases of tularemia, also known as "rabbit fever," with local reporting recently citing a fourth confirmed case.

Tularemia is a rare bacterial illness caused by Francisella tularensis. It can spread through tick or deer fly bites, contact with infected animals, inhaling contaminated dust, or drinking contaminated water. It doesn’t spread from person to person, making it less contagious.

Symptoms can include:

• High fever
• Chills
• Fatigue
• Headache
• Swollen lymph nodes
• Skin ulcers
• Body aches

The illness can be serious, but antibiotics have been effective when cases are identified early.

Hantavirus

Although Hantavirus has been a recent concern, it does appear to be largely contained.

The World Health Organization said a cruise ship-linked Andes hantavirus outbreak involved 13 cases, including three deaths, but that all identified contacts had completed the 42-day follow-up period and the outbreak no longer posed a public health risk.

It’s mainly acquired through contact with infected rodent urine, feces, or saliva, although limited person-to-person transmission has been reported with Andes virus.

Key Takeaway

Measles remains the most worrying current outbreak because of its combination of rapid spread, potential for severe disease and the decline in vaccination rates that public health officials say is helping drive infections higher.

COVID-19 follows because cases are rising across most states.

Cyclospora has the largest known foodborne case count, Candida auris is the most alarming for vulnerable hospital patients, Salmonella is widespread but less severe so far, and tularemia remains serious but localized.

Barclay said the outbreaks illustrate the very different types of infectious disease threats facing public health officials.

Some, such as Cyclospora and Salmonella, are tied to contaminated food supplies; others, including tularemia and hantavirus, are primarily zoonotic infections acquired from animals. But she said diseases that spread efficiently between people continue to pose the greatest long-term public health challenge.

"If I think about COVID-19 or measles, they're highly, highly transmissible between people," she said. "We know from the COVID pandemic that those kinds of human-to-human transmission events are very difficult to control in today's society where we all mix around together."

Contact Newsweek editors for this story: John Fitzpatrick and Robert Greenall

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