The year 2004 was a simpler time to be an infectious disease doctor in the US. Zika and chikungunya hadn’t yet emerged. Mystery RNA viruses weren’t spreading by tick bite around America’s heartland, killing farmers and ranchers. Certainly no one was on the lookout for a meat allergy caused by a tick with a white splotch on its back the shape of Texas. But that was then.

Since 2004, the number of people who get diseases transmitted by mosquito, tick, and flea bites has more than tripled, according to a new report released by the Centers for Disease Control and Prevention on Tuesday. Between 2004 and 2016, about 643,000 cases of 16 insect-borne illnesses were reported to the CDC—27,000 a year in 2004 (the year in which the agency began requiring more detailed reporting), rising to 96,000 by 2016. At least nine such diseases have also been discovered or introduced into the US in that same timeframe. Most of them are found in ticks. Many of them are potentially life-threatening.

What’s to blame for the surge in reported cases? Warmer weather for one thing, said the agency’s director of vector-borne diseases, Lyle Petersen, during a media briefing. Warmer temperatures allow tick populations to expand into new ranges and set up disease reservoirs where none existed before. Earlier springs and later falls also extend the length of tick season, exposing more people to risks longer. And the warmer it gets, the faster mosquitoes can breed and the higher the viral loads they carry around; outbreaks tend to occur when temperatures are higher than normal.

But the CDC report made no mention of climate change, and Petersen, its lead author stopped short of connecting warmer temperatures to the larger global phenomenon. “I can’t comment on why there’s increasing temperatures, that’s the job of meteorologists,” Petersen told reporters on the call. “What I can tell you is increasing temperatures have a number of effects on all these vector-borne diseases.”

Well, what I (with the help of the meteorologists and climate scientists at NOAA) can tell you is that since 1901 the average surface temperature across the contiguous 48 states has risen at an average rate of 0.14 degrees per decade. Eight of the top 10 warmest years on record have occurred since 1998; 2012, 2015, and 2016 were the three warmest. And 97 percent of actively publishing scientists attribute this trend to human activities. So, I think it’s safe to say we do know why there are increasing temperatures.

When asked why the report did not include potential causative factors for the increase in disease transmission, including the impacts of climate change, a CDC spokeswoman said the purpose of the report was only to examine trends in occurrence. "In the US it remains unclear exactly how much climate and weather change may affect the distribution and timing of infectious diseases, as well as the introduction of new diseases," she said. "Research and surveillance is underway that will help address this important question."

It's true that there isn't a scientific consensus linking known climate change trends to observed increases in vector-borne disease transmission. There are a lot of complicating factors at play. Suburban sprawl, for instance: As people have developed wildlands they’ve fragmented habitats for predators like bears, cougars, foxes, and coyotes. With their natural enemies driven out, blood hosts like deer and rodents are thriving, creating large disease reservoirs exactly where people are mostly to be exposed—at the wildland-urban interface.

There’s also an increased volume of international travel and trade coming overseas to the US. That’s how Zika arrived first in Puerto Rico, and then Florida and Texas in 2016.