In the spring, as the wetlands behind the Mosman family home became waterlogged, Keith and his brother Scott would wade barefoot through the shallow pools, seeking out turtles and frogs. This was the 1970s in Raynham, Massachusetts, a time when the town remained largely rural. As they matured, the boys witnessed the transition from apple orchards to shopping centers, and eventually, their father filled in the swamp to construct a swimming pool.

When Keith and Scott had families of their own, their hometown evolved into a suburban extension of Boston. The childhood landscape may have changed, but the mosquitoes persisted, invading every summer from June until the first frost. For Keith and Scott, these insects were merely an annoyance, until a life-altering event occurred last year.

On a September day in 2019, Keith received alarming news from Scott’s girlfriend: his brother had collapsed, suffering a seizure. After being rushed to Morton Hospital in Taunton, Massachusetts, doctors stabilized him but struggled to determine the cause of his condition.

Two days later, Scott was transferred by helicopter to Rhode Island Hospital, where an MRI indicated brain inflammation. A spinal tap confirmed the diagnosis: Scott had contracted the eastern equine encephalitis (EEE) virus from a mosquito bite.

Keith had heard about EEE on the radio during the summer but hadn’t paid much attention. At that time, Massachusetts was experiencing its most severe EEE outbreak in nearly a century. Although transmission is rare, there is no vaccine or treatment for the virus, which has a mortality rate of about 40%. Those who survive often endure long-term neurological issues. Since the virus was first documented in humans in 1938, fewer than 100 cases have been recorded in the state; Scott’s diagnosis marked the tenth case of 2019 with the mosquito season still underway.

The crisis was further complicated by the onset of the COVID-19 pandemic, which began just months later. However, during the summer of 2019, the EEE outbreak—spreading across several states, including Connecticut and Michigan—disrupted daily life in the Northeast. Public health officials recommended staying indoors during dawn and dusk, leading to fear and concern within communities.

Entomologists and epidemiologists monitored the situation closely. Some noted that the season reflected the historical patterns of the virus, while others speculated that climate change could be a contributing factor.

Since the 1980s, scientists have warned that rising global temperatures could expand the range of disease-carrying mosquitoes, increasing the incidence of infections. The Northeast is one of the fastest-warming areas in the U.S., with milder winters and hotter summers fostering mosquito populations. Some experts viewed the 2019 EEE outbreak as an early warning sign of a troubling future.

On September 17, the Mosman family gathered at Scott’s bedside to sing “Happy Birthday,” uncertain if he could hear them or if it would be his last. He was 58, and Keith grappled with a sense of dread.

“I can’t believe that after all our summers outside, we’re now faced with this threat,” Keith reflected. “Is everything really going to be different now?”

Viruses like EEE serve as reminders of humanity's vulnerability. These microscopic entities, which only become active when infecting a host, challenge our civilization’s perceived strength. As humans have dominated nature, we’ve also opened ourselves to new threats, reminding us that our achievements are fleeting and fragile.

As agricultural societies emerged, humans became exposed to zoonotic viruses—diseases jumping from animals to people—leading to infections such as measles and tuberculosis. In urban areas during the Iron Age, trade routes facilitated the spread of novel illnesses, including colds and flus.

During the European colonization, viruses were transported across continents, devastating indigenous populations that lacked immunity. This cataclysmic loss is estimated to have killed up to 90% of Native Americans in North America, a tragedy that may have contributed to a period of global cooling.

As trade and colonization spread, disease-carrying mosquitoes were also introduced to the Americas, causing outbreaks of yellow fever and malaria. The 18th century saw summers referred to as “sickly season,” as citizens sought refuge indoors from the humidity, which was thought to be laden with disease. In 1793, Philadelphia experienced a yellow fever outbreak, leading many, including George Washington, to flee the city. The Reconstruction era also faced devastating outbreaks, such as in Memphis in 1878 when yellow fever claimed around 5,500 lives—12% of the population.

Despite these historical challenges, the mosquito remained an anonymous villain until the late 19th century, when advances in germ theory enabled scientists to identify the viruses that caused illness and the insects that spread them. This understanding led to aggressive mosquito control measures, including draining swamps and widespread pesticide application.

These efforts were so successful that by the post-World War II era, mosquitoes were often seen as mere summer nuisances. This triumph of public health over disease was thought to mark a significant victory over infectious microbes.

However, in 1978, Theodore Andreadis began tracking mosquito populations at the Connecticut Agricultural Experiment Station, discovering that EEE emerged from the red maple and white cedar swamps of the Northeast. Migrating songbirds, immune to the virus, would visit these wetlands, allowing mosquitoes to transmit the virus among them.

Because the primary mosquito species, Culiseta melanura, had a limited flight range and primarily fed on birds, the virus typically remained confined to the swamps. However, when conditions were right, the virus would spread to other mosquito species capable of biting mammals, including humans, leading to sporadic outbreaks.

The first known infection in Massachusetts occurred in 1831, when farmers found horses behaving erratically before dying. The first human case was recorded in 1938, resulting in 25 fatalities.

Andreadis meticulously documented the patterns of EEE over the years, observing that outbreaks were unpredictable, alternating between years of heightened activity and years of silence.

However, in 2003, the trend began to shift, with increasing human cases reported across the Northeast. The virus was creeping north, and Andreadis hypothesized that climate changes were creating favorable conditions for mosquito populations and EEE transmission.

Weather patterns indicated milder winters and warmer summers, encouraging the survival and breeding of mosquitoes. This aligns with predictions that a warming climate would allow disease-carrying mosquitoes to expand their range, leading to increased infections.

In 2013, Andreadis and a colleague published findings linking the rise in EEE outbreaks to climate change, coinciding with an exceptionally hot summer in Connecticut. For the first time, EEE-related deaths were documented in the state.

For humans, mosquitoes remain the deadliest creatures on the planet, responsible for approximately one million deaths annually. While malaria is the most lethal mosquito-borne disease, affecting predominantly sub-Saharan Africa, EEE's localized impact has historically made it a low priority for medical intervention, limiting efforts to develop vaccines.

Public health departments manage EEE outbreaks through surveillance, particularly in the spring when mosquito populations begin to thrive. In July 2019, Catherine Brown, Massachusetts state epidemiologist, received reports of EEE detected in mosquito samples, indicating a potential outbreak. Aerial spraying and public warnings followed.

The first confirmed human cases of EEE in Massachusetts occurred in August, with symptoms escalating rapidly. Some patients recovered, while others faced severe neurological consequences.

Scott Mosman initially showed signs of recovery, but his condition deteriorated, and he was moved to hospice care, ultimately becoming one of the four EEE fatalities in Massachusetts that year.

As neighboring states reported increasing EEE cases, public anxiety grew, compounded by misinformation on social media. Communities took measures to mitigate mosquito exposure, leading to cancellations of outdoor events and changes in daily life.

As the mosquito season ended with the frost in November, the Northeast faced a new challenge: the emergence of COVID-19. This new virus, unlike EEE, could spread between humans, leading to a global health crisis.

Despite their differences, both outbreaks exemplified the interconnectedness of our world and the impact of a changing environment. In January, Andreadis reflected on the ongoing challenges posed by EEE, noting that the resurgence of the virus was linked to the health of local swamps and changing land use.

As wetlands have rebounded, suburban encroachment has introduced more human activity into these ecosystems, increasing the likelihood of EEE transmission. This reality serves as a reminder of the delicate balance between human development and environmental health.

In light of these challenges, public health officials remain vigilant, preparing for the possibility of future outbreaks as communities navigate the complexities of a changing world.