Although polio has largely receded from public health concerns in the U.S., where it was eliminated more than four decades ago, strains of the virus that causes the disease still circulate in parts of Europe, Asia and Africa. At home, measles outbreaks have recently been reported in Pennsylvania, Virginia and Utah, pushing the total number of cases in the U.S. during 2026 to more than 2,220—already more than last year’s count, according to the Centers for Disease Control and Prevention.

“More than anything, it’s a good reminder that staying up to date on vaccinations is critical to staying safe and healthy while traveling,” said Michael Ben-Aderet, MD, associate director of Hospital Epidemiology at Cedars-Sinai.

The Cedars-Sinai Newsroom spoke with Ben-Aderet about what travelers should know about polio and measles and how to stay safe this summer.

Where does polio still exist?

Polio is caused by a highly contagious virus that has essentially been eliminated from most of the world through vaccination. Wild strains still persist in a handful of countries, including Afghanistan and Pakistan, which means the vast majority of international travelers are at relatively low risk, particularly if they’ve been vaccinated. In recent years, roughly 30 countries in Africa, Europe and Asia, including the U.K. and Germany, have shown signs of the virus circulating. However, those signs are not necessarily tied to outbreaks. For example, public health officials in the U.K. have detected the virus in wastewater tests, which likely points to some degree of asymptomatic polio.

How does polio spread?

The poliovirus typically spreads when an infected person doesn’t wash their hands after using the restroom and contaminates water, food or other surfaces that others touch. It usually causes a mild illness that most people recover from quickly. What makes polio historically frightening is that a small percentage of infections can progress to paralysis, typically affecting the legs and feet, when the virus invades the nervous system. That’s why vaccination is so important. It has essentially eliminated that fear in populations where immunization rates are high.

How big of a threat is polio to international travelers?

Almost everyone in the U.S. received the polio vaccination series as a child. For vaccinated travelers, the risk is minimal. For older adults concerned that their immunity may have waned over the years, a booster is worth considering before visiting regions where polio transmission remains a possibility. The polio vaccine has minimal side effects, so there is little harm in getting the vaccine. You won’t receive the full benefit of immunity until four weeks after vaccination, but you’ll start receiving some protection immediately.

Where is measles a concern?

Measles is, in many ways, a more immediate risk to summer travelers, especially in crowded or enclosed spaces. It’s far more contagious, spreading through airborne particles from an infected person’s cough or sneeze, and it can stay in the air for up to two hours after that person leaves.

Recent clusters have emerged in parts of California, Texas and Oklahoma, and the largest outbreaks have been reported in India, Bangladesh, Yemen, Mexico and Pakistan. Measles outbreaks are happening in multiple countries throughout the world. You could try to avoid known measles outbreaks as you travel, but the chance of encountering someone with measles has risen as fewer people get vaccinated, and you don’t know when you’ll be exposed.

How protected are those who have been vaccinated?

Most fully vaccinated individuals are well protected for life. Adults who are uncertain whether they ever received the two doses of the MMR vaccine, which protects against measles, mumps and rubella, should consider getting at least one dose. Adults at high risk should consider two doses at least 28 days apart. Children are recommended to receive two doses: one at 12 to 15 months and one at 4 to 6 years old. Babies 6 to11 months old should get one dose before they travel and two more doses after their first birthday.

It’s always a good idea to check your vaccination records and to consult your primary care provider or a travel medicine clinic four to six weeks before traveling abroad.

Read more from Cedars-Sinai Stories and Insights: 8 Expert Tips to Avoid Getting Sick When You Travel

The U.S. experienced a high number of COVID-19 cases this summer, and another surge is expected this winter.

Ben-Aderet told Wake Up Call host Heather Brooker that most people should get the updated vaccine now because they may not have had one since the last vaccine update in fall 2023.

“If you’ve been vaccinated very recently or were recently infected with COVID, there may be some reasons why you would want to delay that shot a little bit,” Ben-Aderet told Brooker, adding that people with medical conditions or other concerns should discuss timing with their doctor.

Ben-Aderet told Brooker it’s safe to get vaccines for COVID-19, the flu and RSV at the same time.

“That’s what I do every year—I get my COVID shot and my flu shot together,” he said. “There’s no interaction between the two vaccines. It’s a very, very different type of virus—it’s actually a different type of vaccine—and so there’s really no medical reason that you would want to delay them.”

Ben-Aderet told Brooker that vaccine reactions, like a sore arm or a fever, are normal immune system responses.

“Nobody likes getting a shot, right?” he said. “And a lot of people do feel bad the next day. But that being said, that’s a heck of a lot better than getting COVID. There’s really no comparing the two.”

Click here to listen to the segment, which begins at 25:15, from KFI-AM.

That’s because global warming causes temperatures to rise which, in turn, expands the reach of insects carrying infectious diseases. Species of mosquitoes that thrive in warmer environments are finding newly hospitable homes in places where the climate is warming and the summer heat lingers longer. As a result, physicians in once mild parts of the U.S. are seeing increasing cases of mosquito-borne diseases, such as Zika virus, West Nile virus and now dengue fever. 

“Warmer weather means warmer pools of standing water, a friendlier environment for breeding mosquitos, especially without very cold winters to kill off the eggs and larvae,” said Kimberly Shriner, MD, medical director of Infection Prevention and Control at Huntington Health, an affiliate of Cedars-Sinai. “There is no question that we’re seeing more Aedes mosquitos—the species that carries dengue—because of hotter temperatures.”

Another cause: People are traveling more widely and frequently, said Michael Ben-Aderet, MD, associate medical director of Hospital Epidemiology at Cedars-Sinai. He explained that this increases the odds that a traveler returns home with a mosquito-borne infectious disease, gets bit by one of the many Aedes mosquitos now thriving in Southern California, and that bug carries that disease to someone else.

“Dengue is well known in tropical places like South America, Central America and Puerto Rico,” Ben-Aderet said. “We’ve always had dengue cases in Los Angeles linked to travel, but now we’re encountering patients who haven’t traveled at all—they’ve never left Los Angeles. These individuals were bitten by mosquitoes locally. This suggests that there are likely more mosquitoes carrying dengue that we haven’t detected yet, partly because we’re not testing frequently enough and many people with dengue don’t become very ill.”

He added that Southern California is a perfect environment for dengue transmission. “We have mosquitos, a big population and people are traveling more than ever before,” he said.

Breakbone Fever

The Aedes mosquitos that can spread dengue, as well as Zika virus, started showing up in Southern California about a dozen years ago. While most people are familiar with the ankle-biting tendency of these mosquitos, they’re likely less aware of dengue and the disease’s symptoms.

Dengue can cause a wide range of symptoms from none at all to very severe, Ben-Aderet said. It starts with a high fever and severe muscle pain, giving it the nickname “breakbone fever.”  Dengue often appears with a diffuse red rash that blanches, turning white or the color of one’s skin after pressing on the rash and releasing pressure. Patients can also experience gastrointestinal symptoms like nausea.

The second time someone gets dengue, it can be much worse, said Shriner, who has treated patients with disease during annual medical mission trips to Tanzania with a nonprofit that provides clinical care and medical education. Dengue is a hemorrhagic fever, like the Ebola virus, that can cause bleeding—in this case, bruises bloom under the skin from “leaky” blood vessels.

For mild cases, treatment is supportive care: fluids, rest and Tylenol. More severe cases might need intensive care in a hospital. Highly specialized vaccines exist, but they’re primarily intended to help children avoid a dangerous second infection in places like the Philippines where the disease is endemic.

Canary in the Coal Mine

For local physicians, the spread of tropical diseases in once milder climates means they must think more broadly when diagnosing a sick patient.

"Whenever I see a patient in today's world, I need to keep a very open mind about what they might have," Ben-Aderet said. He added that the sporadic dengue infections we’ve seen from area mosquitos indicate we don’t yet have a “reservoir,” a sustained source of infections to make dengue endemic to Los Angeles. But it could happen in the not-too-distant future, he said.

Shriner emphasized that she doesn’t want people to panic. “We should be worried more about COVID-19 and the flu,” she said.

Still, she cautioned that smaller, novel outbreaks—like the emergence of dengue in Los Angeles—are “canaries in the coal mine” sending a message that we need to pay attention.  

“We live in an interconnected world with a fragile climate that is changing,” Shriner said. “We want to approach this scientifically, compassionately and with humanity. All of us are at risk for these diseases; all of us need to find solutions together.”

Read more on the Cedars-Sinai Blog: Everything You Need to Know About Bug Bites