(CNN) As director of the Centers for Disease Control, Dr. Tom Frieden has tweeted and talked about everything from AIDS to obesity to mosquitoes in Hawaii, the overuse of antibiotics, polio in Pakistan, Ebola, sanitation services in East Africa, ambulances in Sierra Leone, premature babies, sexually transmitted diseases, flu shots and World Toilet Day.
A prolific communicator, it's hard to think of a health topic Frieden hasn't discussed in his six years on the job --except the second-biggest killer of young people in the United States. That one he's been quite silent on.
It's guns.
In a review of Frieden's tweets, videos, blogs, CDC news releases and public appearances, he doesn't mention guns and the lives they take.
It's not that the CDC doesn't research violence and injuries. It does. The Division of Injury Prevention and Control examines the tens of thousands of American lives lost every year not to diseases but to causes ranging from gang violence to domestic abuse.
So why is the nation's leading public health official, a nonstop messaging machine, not talking about something that kills more than 30,000 people a year in the United States? That's more than the number of Americans who die of AIDS or colon cancer or prostate cancer. That's more than the number of people who died in the entire international Ebola outbreak last year.
Related: U.S. Gun Violence: The story in charts and graphs
Why is he silent about guns, when they kill 10,178 people between the ages of 13 and 30 each year? That's more than drownings and cancer combined for that age group, and nearly as many as motor vehicle accidents, according to the CDC's own data.
"He's chicken," said Andrew McGuire, executive director of the Trauma Foundation, an injury prevention group in San Francisco. "He suffers from gun-itis. It's a disease where you're scared to death of the gun lobby. That's what's really going on. If you have to shut up about something that is so murderous, it doesn't make any moral sense to me."
Frieden declined to be interviewed for this story. A statement from the agency to CNN noted that the CDC keeps track of firearm deaths and publishes studies, finding that "more than 117,000 Americans are non-fatally injured or die each year from a gunshot wound, making firearm-related injuries among the five leading causes of death for people aged 1-64 in the United States."
The statement did not explain why Frieden has chosen to be quiet about this leading cause of death. Frieden's defenders say he's not chicken but wise.
They say if he dares publicly talk about lives lost to guns, Congress might retaliate by taking away CDC funding for a whole host of safety issues, including motor vehicle accidents, traumatic brain injury and child abuse, to name just a few.
That, they say, is the strength of the gun lobby in the United States.
On the morning of December 14, 2012, Linda Degutis stepped out of a meeting at the Atlanta Marriott Century Center to make a phone call in the lobby. She looked up at the television to see reports that children had been shot at Sandy Hook Elementary School in Newtown, Connecticut.
The shooting horrified everyone, but it hit Degutis especially hard. She'd lived in Connecticut, taught at the Yale School of Public Health and had been director of the state's public health workforce training initiative.
As a nurse involved in public health, this was exactly the type of atrocity she was supposed to help prevent.
At the time of Sandy Hook, Degutis was living in Atlanta, directing the CDC's National Center for Injury Prevention and Control.
Now is the time, she thought, as she rushed back from the hotel to her office at CDC headquarters. With schoolchildren dead, the CDC would surely start doing research again into the causes of gun violence.
In the mid-1990s, members of Congress accused the CDC of being biased against gun ownership and using intentionally flawed research methods. Congress yanked the agency's funding for gun violence research.
CDC research had helped bring down death rates from motor vehicle accidents and smoking, among many other major preventable causes of death. If the agency started to do research again into the causes of gun violence, perhaps lives could be saved, Degutis thought.
At the very least, maybe her boss, Frieden, would finally speak out about the horrors of gun violence. After all, 20 little children had just been slaughtered.
Months passed. No research was done. Frieden didn't speak out.
"It was very frustrating and disappointing," she added. "In public health, you're very action-oriented, you want to figure out how to keep something bad from happening again."
Four months later there was a ray of hope. President Barack Obama had ordered the CDC to come up with a list of research priorities on firearm violence.
"We need to understand the problem of gun violence and identify effective gun violence prevention strategies to help keep people safe," Degutis said on April 23, 2013, as she set forth the CDC's charge to the Institute of Medicine, a part of the National Academy of Sciences.
The panel worked quickly, producing a report in just six weeks. The Institute of Medicine issued a news release and trumpeted the report on its website. Journals from Preventive Medicine to the Journal of American Physicians and Surgeons wrote about the new report.
But not the CDC. The day after the report was issued, on June 6, 2013, the agency issued a news release on how to stay safe in the summer heat.
The very agency that had commissioned the report, Degutis' own employer, was silent about it. Her hopes were dashed. "You can't win a battle if you're just watching it," she said.
In frustration, Degutis resigned her position at the CDC six months later.
Two and a half years after it was issued, the Institute of Medicine's report is just another document in government archives, the research agenda it espoused never adopted by the agency that requested it.
Here are just some of the many questions to which we still don't know the answers:
-- Would a federal assault weapons ban save lives or cost lives?
-- State laws allowing people to carry concealed weapons are supposed to deter crime, since attackers never know who's carrying a gun. Have these laws worked out that way?
-- If gun owners had to register their weapons with the government, would fewer people die from gun violence? Would more die?
-- Are certain types of people more prone to gun violence than others? If the answer is yes, what's the best way for authorities to reach out to these people?
-- Do background checks help save lives? What questions, if any, should be asked of someone who wants to buy a gun? Questions about the person's criminal record? Their psychiatric record? Something else?
Years ago, there were many such unanswered questions about motor vehicle accidents. Do seat belts save lives or do they trap people in burning cars? What engineering change might make cars less likely to flip over? What's the best way to design road signs so drivers actually pay attention to them?
Research helped answer these and many other questions. Cars, roads and laws were changed, and from 2004 to 2013, motor vehicle deaths plummeted nearly 25%, according to the National Highway Traffic Safety Administration.
"Because we haven't been able to do [gun violence] research for 20 years, we've missed a lot of opportunities to reduce homicides and deaths," said Dr. David Satcher, who served as U.S. surgeon general and as director of the CDC. "We're all suffering from not having this research. It's shameful."
In its statement to CNN, the CDC agreed that "public health research is fundamental to understanding the problem and developing scientifically sound solutions," adding that Congress has not given money to do such research.
In contrast to Frieden, Satcher spoke very publicly about guns while he was in office from 1993 to 2002, writing an op-ed piece in The Washington Post and testifying to Congress.
"It was important to me to talk about anything that was a major threat to the lives of people," he said.
But in an interview with CNN, he emphasized that he doesn't blame Frieden for his silence. Congress clearly doesn't want the CDC involved in gun research. Every year since Sandy Hook, Obama has asked for money to fund such research, and every year Congress has turned him down.
If Frieden were to speak out about guns, Satcher said, Congress might take away CDC's funding for other kinds of research. "I'm not being critical of the leaders of the CDC," Satcher said.
Dr. Thomas Farley agrees. The former commissioner of health for New York City and a former CDC medical epidemiologist said it's a "reasonable position" for Frieden to stay silent on gun violence research.
Twenty years ago, encouraged by the NRA, members of Congress threatened to cut the entire CDC National Center for Injury Prevention and Control because of gun violence research. That division works to reduce deaths from motor vehicle accidents and other types of injuries that kill 180,000 people each year.
Cutting that center, he says, would have been devastating.
"That was a very real threat, and now Congress is even more aligned with gun interests," said Farley, who served as New York City's health commissioner from 2009 to 2014. "Frieden has to be concerned about all public health problems, not just one."
Farley said he's not sure what he would do if he were in Frieden's position.
"I'd look at all the causes of death and see where do I have the opportunity to make a difference by speaking up and where might I actually make things worse. Then I'd do the calculus," he said.
Talking about guns doesn't just potentially threaten funding. It could also be career suicide.
"When I was at the CDC, I did speak out and tried to protect the science, but I basically got fired for it. They said 'You've got to go'," said Dr. Mark Rosenberg, director of the CDC's National Center for Injury Prevention and Control from 1994 until 1999.
Like Satcher, David Hemenway, professor of health policy at the Harvard School of Public Health, said he understands why Frieden has stayed silent about guns.
"After [the CDC] got beat up so badly in the mid-1990s, they're rightfully afraid to do anything. They're afraid to say the words 'guns' or 'firearms'," he said.
Would he talk about guns if he were director of the CDC?
He paused.
"I would like to think I would. I like to think I'd be a brave person, but I'm not so sure," he said.
Andrew McGuire, for one, would like to see members of Congress threaten to destroy parts of the CDC.
"Good! Good! Nothing would be better than to show how outrageously out of control the gun lobby is and their puppets in Congress," the head of the Trauma Foundation said. "It would be a tipping point to show how crazy their power is."
A spokeswoman for the National Rifle Association did not respond to requests for an interview for this story. A post on the organization's legislative action website refers to the CDC as having a "history of rabid anti-gun 'research' " and "of conducting 'junk science.' "
Another NRA post questions the extent of gun violence. "Gun control supporters in the public health field claim that gun violence is an 'epidemic,' but gun violence is alien to most people's experiences," according to a statement on the NRA's website.
McGuire said despite the NRA's influence in Congress, Frieden should speak publicly about the lives lost to guns. "If you shy away from tough problems, what the hell does that say about you as a public health advocate?" he asked.
He points out that in the 1980s, the CDC was on the forefront of the fight against AIDS even when politicians were afraid to say the word. He wonders why Frieden can't "have that same courage" with gun research.
McGuire said if he were in charge, he'd have Frieden put on his U.S. Public Health Services uniform and publicly ask Congress to restore CDC funding to do gun violence research.
Two former directors of the CDC's injury prevention division -- Linda Degutis and Mark Rosenberg -- agree that Frieden should speak up. They say maybe it would persuade Congress to fund research on gun violence and maybe that research would encourage policy changes that save lives.
They point to a long history of this effect: motor vehicle deaths declined after research led the way to effective strategies; the spread of HIV was curbed as the CDC studied the epidemic; and tobacco-related deaths declined as the CDC helped figure out the best messages to reach smokers.
Not everyone is displeased with Frieden's silence.
Dr. Timothy Wheeler thinks it's entirely appropriate. Director of the group Doctors for Responsible Gun Ownership, Wheeler said the CDC revealed itself to be "fundamentally biased against gun ownership" more than 20 years ago, and its employees should remain silent.
Wheeler said CDC researchers used flawed scientific methods to cast gun ownership in a bad light.
He points to studies such as a 1993 New England Journal of Medicine report showing guns didn't confer protection, but rather made it more likely there would be a homicide in the home.
He also criticized a study done that year by Rosenberg and four other CDC leaders and published in the journal Health Affairs. In it, the CDC authors stated that restrictive licensing of handguns is one of "a broad array of potentially effective intervention strategies through which violence might be prevented."
Later the report mentioned a "critical need to assess the value of the numerous intervention strategies that can be, and in some cases, are being adopted." One of those strategies was to reduce the number of guns in the United States by prohibiting gun ownership.
For Wheeler, those two statements amount to a "political agenda."
"This is a political agenda masquerading as scientific research about guns," he said. "An institutional bias like this is impossible to eradicate. The taint of the CDC's mission against gun ownership is indelible. It can't be removed," he said.
But there is some common ground in this polarizing debate. Rosenberg and Wheeler, on opposite sides of the issue, both want to stop people from dying from gun violence.
Wheeler thinks criminologists are best suited to figure out how to make that happen. Rosenberg said he doesn't have a problem with criminologists doing gun violence research, but he thinks Frieden and his staff at the CDC are also well-suited to do the research. After all, the CDC helped come up with strategies to save lives lost to HIV and tobacco and to car accidents.
Rosenberg noted that even his former foe in the CDC gun research debate, Rep. Jay Dickey, the Arkansas Republican who in 1996 sponsored the congressional ban on certain types of gun research, now wants his legislation reversed.
"[Frieden] needs to let people know that the CDC can help find answers that will keep communities safe but also let law abiding gun owners keep their guns," Rosenberg said.
But to do that, Frieden needs to talk about guns, he said. "I think leaders should lead. It's the only way out of this mess we're in."