As diseases go, Zika virus was always considered minor league.

It didn’t make people all that sick; most infected people had no symptoms at all. Zika was confined to a relatively narrow belt that ran from equatorial Africa to Asia.

Today, Zika has spread to Central and South America and is linked to an alarming increase in once-rare birth defects in Brazil. Although Zika was first diagnosed in Brazil in May, it’s been linked to more than 3,500 cases of microcephaly, in which infants are born with small heads and immature brain development.

Yet Brazil isn’t just fighting Zika.

That country is also combating outbreaks caused by dengue and chikungunya viruses, which are known for causing fevers and debilitating joint pain. Dengue can be fatal.

The World needs to prepare for a similar scenario, in which epidemics of multiple mosquito-borne diseases break out simultaneously, according to Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, who co-wrote a new report in The New England Journal of Medicine.

Diseases spread by insects “are the next big threat to the Western Hemisphere, including the U.S.,” said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston.

Dengue, chikungunya and Zika are spread by the same species of mosquito – known as the Aedes – and can be found in much of the USA, said Amesh Adalja, a senior associate at the Center for Health Security at the University of Pittsburgh Medical Center.

And while mosquitoes have brought disease and death for thousands of years, modern life is magnifying their reach, wrote Fauci and co-author David Morens, also of the National Institute of Allergy and Infectious Diseases, in a report published online Thursday.

“Urban crowding, constant international travel and other human behaviors … can cause innumerable slumbering infectious agents to emerge unexpectedly,” they wrote.

Parts of the USA already have been hit.

Dengue has infected more than 200 people in Hawaii.

Chikungunya – which was unknown in the Western Hemisphere until 2013 – now spreads routinely in Puerto Rico and the U.S. Virgin Islands. In December, Puerto Rico reported its first locally acquired case of Zika, according to the Centers for Disease Control and Prevention.

This week, health officials announced a Houston-area woman was diagnosed with Zika, which she developed after traveling to Latin America.

For reasons that scientists don’t understand, Zika outbreaks spread by Aedes mosquitoes often follow epidemics of chikungunya, Fauci and Morens wrote.

Modern medicine doesn’t have much to offer against these illnesses.

Although Sanofi Pasteur has developed a dengue vaccine, there are no vaccines for chikungunya and Zika. There are no drugs to fight any of these diseases. Because they’re caused by viruses, antibiotics don’t work against them.

Rather than tackle each disease individually, researchers need to develop drugs that treat “whole classes of viruses,” Fauci and Morens wrote.

But just diagnosing diseases as new as Zika can be difficult. There are no commercial tests to diagnose it, so blood samples have to be sent to labs capable of running sophisticated tests.

In many ways, the USA’s wealth protects it from mosquito-borne diseases, Adalja said. The USA eliminated malaria in the mid-20th century by draining swamps where mosquitoes bred and killing them with pesticides such as DDT.

Today, air conditioning allows Americans to spend more time indoors, where they’re protected by window screens, Adalja said. In Texas, dengue spreads far more commonly on the Mexican side of the border than on the U.S. side, even though residents live just a few miles apart.

In developing countries, controlling mosquitoes is more difficult. Many people live in crowded slums without drainage or sanitation – conditions that are still found in some parts of the USA, such as the Gulf Coast, Hotez said. Climate change is also allowing tropical diseases to move north.

Researchers “urgently” need to better understand these viruses, Fauci and Morens wrote. In particular, researchers need to study why Zika virus has spread so far, so quickly, and why it seems more dangerous than in the past, Adalja said.

In their paper, Fauci and Morens noted: “We clearly need to up our game with broad and integrated research that expands understanding of the complex ecosystems in which agents of future pandemics are aggressively evolving.”