Changes in weather patterns are affecting how diseases spread around the world. Diseases like Chikungunya were once limited to tropical regions. However, they are now appearing in places like California. Clinicians in these newly affected areas might lack experience with these diseases. This could lead to delayed diagnosis and treatment.
The following section examines these expanding infectious threats and their growing impact.
1. Dengue
Dengue cases are hitting new highs in the U.S., mostly in Florida, Texas, and Hawaii. Over 6,000 local cases were reported in 2024, the most since 2013.
The number of people infected with dengue is likely to be much higher. Only about 25% will show symptoms.
Warmer temperatures are helping Aedes mosquitoes—the insects that spread dengue—move into new areas. This is allowing the disease to spread to more places than before. By 2050, these mosquitoes could reach even northern cities like Chicago.
2. Lyme disease
Over 42,000 cases of Lyme disease were reported in the United States in 2017.
Ticks that carry Lyme disease are also thriving in colder parts of Canada. Between 2009 and 2021, Canada reported a nearly fivefold increase in Lyme disease cases. They are appearing in Quebec and Ontario where they didn’t exist 20 years ago.
Warmer conditions are favoring tick survival and expansion. Rising temperatures have also extended the tick season by 3–4 weeks.
Untreated Lyme disease progresses from a mild rash to severe neurological, cardiac, and arthritis complications. It becomes increasingly difficult to treat at each stage.
Lyme disease cases could cost public health systems over half a billion dollars a year over the next 20 years.
Lyme disease is an example of climate change pushing disease-carrying species into new areas. This pattern could repeat with other diseases and add to public health burden across North America.
3. West Nile virus
West Nile virus is more than just a fever.
A senior physician and immunologist in the United States suddenly began forgetting simple words. He was asking his family questions he should have known the answers to. When he was hospitalized with dangerously high fever, he was initially treated for sepsis. The real culprit, however, was West Nile Virus.
West Nile virus spreads through mosquito bites and nearly every U.S. state has reported cases. Warming temperatures are changing mosquito seasons, causing them to start earlier and end later. This gives mosquitoes more time to transmit the virus to humans.
Most cases are asymptomatic. However, 1 in 150 cases develop a neuroinvasive disease. Patients can develop altered mental states, tremors, or movement disorders.
In Europe, the virus is showing up more frequently, especially after very hot weather. Heat waves have caused West Nile virus to spread 40-65% more in southern European countries. Greece used to have very few cases, but now hundreds of people get sick each summer after heat waves.
Climate change is making this problem worse by causing more frequent and intense heat waves. When temperatures rise above 86°F (30°C), the virus multiplies faster inside mosquitoes. This makes the mosquitoes more infectious and increases the chances they’ll spread the disease to humans.
4. Malaria
Recently, for the first time in 20 years, people with no travel history contracted malaria in Florida and Texas. This means that mosquitoes living in the United States can now spread malaria, which is a serious health concern.
Since the mid-1970s, cases of malaria have been growing in the United States.
Climate change and warmer temperatures let malaria mosquitoes live in places that used to be too cold for them. With time, more communities will face malaria threats. This includes places that never had malaria before — such as North America and Northern Europe.
Experts expect that this will put pressure on health systems and make healthcare more expensive. For example, a study in the United States found that patients with severe malaria paid two to four times more than those hospitalized with uncomplicated malaria.
5. Leptospirosis
Leptospirosis is a bacterial infection that spreads through contaminated water. It is particularly concerning because its initial symptoms, such as fever, headache, and muscle aches, are non-specific. These symptoms can be mistaken for common illnesses like the flu or viral infections. This often leads to delays in diagnosis and treatment, increasing the risk of complications.
If left untreated, leptospirosis can progress to severe organ failure. It can include life-threatening conditions such as kidney and liver damage. In severe cases, leptospirosis can result in significant health complications or even death.
Leptospirosis sees major spikes after flooding events because the bacteria spreads through infected urine from animals. This includes livestock, wild animals and pets. For instance, recent studies show nearly 15% of dogs in the Midwest and Northeast already carry the infection.
For example, instances of hurricanes or heavy rains are growing in the midwest. During extreme weather humans and animals might shelter together. Local and private water sources like wells and tanks might get infected with animal urine during this period. Without access to treated water people might get infected with leptospirosis.
6. Zika
Zika virus is not very well known, but could lead to severe illness. The virus is spread through Aedes mosquitoes. Many people who catch Zika virus might not even know they’re sick. Some experience only mild symptoms.
However, when pregnant women become infected, the virus can cause major birth defects in babies. These include microcephaly, a condition where a baby’s head is smaller than expected, and other neurological problems.
In rare cases, the virus can cause Guillain-Barré syndrome, a serious condition that makes muscles weak and can lead to temporary paralysis.
Climate change is making the situation more complicated. As global temperatures rise, Aedes mosquitoes are expected to live in more areas of the United States. This means more regions could face Zika outbreaks in the future.
7. Chikungunya
Chikungunya was first reported in 2013. In just 10 years, over 3.6 million cases of Chikungunya were reported across the Americas. This virus has spread to more than 100 countries, including regions in the Americas, Africa, Asia, Europe, and the Indian and Pacific Oceans.
Doctors sometimes find it challenging to identify the infection because its early symptoms, such as fever and joint pain, can be similar to other diseases. Some patients experience ongoing joint pain that can last for months or even years after the initial infection.
Climate change is making the situation more complex. The virus is primarily spread by the Aedes albopictus (Asian tiger mosquito), which has expanded its living area by about 25% in the past ten years. This expansion is particularly noticeable in Europe and North America.
Between 2007 and 2022, researchers documented 4,730 Chikungunya cases across twenty-two countries in mainland Europe.
The combination of frequent travel, the presence of at least two types of mosquitoes that can carry the virus, and a population with little previous exposure creates conditions that could lead to future virus outbreaks.