While the past decade has seen great strides in medical technology, it hasn't seen solutions to all of our health problems.
Some of these threats turned out to be almost nonexistent. Others were arguably overblown. Some caused widespread harm.
So what new threats have been robbing you of sleep since the annual odometer rolled over from 1999 to 2000? Join us as we take a look at the top ten new threats of the last ten years.
Swine Flu (H1N1)
Since it came to public attention in the United States in April, the largest health scare of 2009 has been swine flu.
While other forms of the virus typically peak in February and largely affect the elderly, this strain of the H1N1 flu virus came out of season and mostly affected younger people.
In June, the World Health Organization declared the virus a pandemic, meaning that it was widespread on multiple continents.
Manufacturers began producing vaccine at the end of the spring, but there were shortages nationwide, even late into the fall.
While the majority of cases of the flu have been mild, thousands of American deaths have been attributed to the virus.
But no matter the severity, many health experts agree there are lessons to be learned.
"We just have to note, and this was a bit scary, that when H1N1 came along in communities... our capacity to take care of [patients] was stretched," said Dr. William Schaffner, Chair of the Department of Preventive Medicine at Vanderbilt University Medical Center. "We don't have a lot of reserve in the health care system anymore."
Bisphenol A (BPA)
While much of the alarm over the chemical bisphenol A, or BPA for short, has come lately, scientists have been looking at it for years.
In January of 2000, an article in the Journal of the American Dental Association discussed how BPA -- which is used in some dental sealants -- was not found at detectible levels in the body more than a few hours after the treatments.
Since then, studies have shown the chemical to cause birth defects in lab animals, and even create some problems in humans in high doses. The chemical, used in household plastics, has also been found in babies, leading to increased scrutiny from regulatory bodies.
But while we know it is present in humans and can create problems at high levels, it remains unclear what the effect of BPA is in humans at lower doses.
"I would say that there's growing evidence that it is a significant concern, but it's not clear yet how much of a concern," said Joel Schwartz, a professor of epidemiology at the Harvard School of Public Health and director of the Harvard Center for Risk Analysis. "There's a lot of things that still need to be understood, but there's certainly enough things to say, 'Yes, this is something that needs to be on our radar screen.'"
Lead Paint On Toys From China
In 2007, a number of products made in China were recalled -- but perhaps the recall that drew the most attention was of children's toys containing lead paint, including some from the popular Thomas the Tank Engine line.
The problem wasn't so much one of scientific analysis as it was of enforcement.
"We do know that lead is bad for you," said Schwartz. "Kids and toys are a bad place to put that exposure together. That's a case where that's just outrageous."
The exact effects of the oversight are unknown, but it did shine a spotlight on imported goods.
"It's doing a little more to make sure this stuff doesn't keep slipping in," said Schwartz.
Concern over trans-fats -- found in such crowd-pleasing but doctor-disapproved foods as doughnuts and French fries -- came to a head in 2006, when New York City became the first city to ban trans-fats from restaurants.
"The issue became viral, and a lot of it was related to population studies that came out of Harvard University," said Keith-Thomas Ayoob, director of the nutrition clinic at the Albert Einstein College of Medicine. "The problem with them is they tend to raise bad cholesterol and lower good cholesterol."
In response to doctors' concerns, most trans-fats have been removed from food products -- but in many cases they may have been replaced by saturated fat, which can also be harmful in excess.
Ayoob said trans-fats were "an easy whipping post."
Trans-fats may have disappeared because they were easy to replace with other ingredients. And ultimately, the virtual disappearance of trans-fats may be a better testament to the power of economics in responding to consumer demand than that of regulation responding to public complaints.
"It's box office poison now, is what it is. No one really wants to list that on an ingredient list," said Ayoob. "That's one where the food industry responded much more quickly than government ever would have. It just didn't pay to keep it in there."
Bird Flu (H5N1)
This year's swine flu pandemic wasn't the first time Americans were concerned with a strain of flu named for an animal.
At the beginning of the decade, avian influenza was a concern in Southeast Asia because of the devastation it was causing in chicken populations. But concerns soon arose about its spread to humans and the possibility it would mutate into a form that could spread from person to person.
"These new influenza viruses usually are modified viruses that come from birds, and now, we know, swine," said Schaffner. "We know that influenza viruses change on an annual basis... The world's population will be or will virtually be completely susceptible."
But concerns over avian flu did have one positive effect for the flu vaccine industry, which has been maligned for its reliance on old technologies to create the vaccine each year. Because antigens for flu vaccines are grown in chicken eggs, it was hard to develop a vaccine for a virus that was deadly to birds, and so work had to be done to begin developing a means of creating antigen without using eggs.
Although the new manufacturing processes are not available yet, "It was exactly H5N1 bird flu that stimulated a number of new ways to create new vaccine," said Schaffner. "What we see now actually came forward as a consequence of all that concern with H5N1."
Severe Acute Respiratory Syndrome (SARS)
SARS was the first novel virus to captivate the world's attention this decade after it was identified by the World Health Organization in February 2003.
The respiratory infection was first reported in Asia and then spread to North America, South America and Europe before being contained.
Like influenza, the virus could spread through airborne particles, but it was far more deadly when it infected someone. According to the WHO, 8,096 people were infected worldwide. 774 people died. The virus receded by the end of 2003.
"SARS-like infections, I think, epitomize the emerging infectious diseases," said Schaffner. As for whether the strain could re-emerge, he said, "My crystal ball is pretty cloudy about that... Trying to anticipate whether it would come back or not would be very, very difficult."
Methicillin-Resistant Staphylococcus Aureus (MRSA)
MRSA is the best-known of a number of bacteria that resist many of the antibiotics used to treat them. The emergence of MRSA and other bacteria has been blamed on a combination of heavy use of antibiotics and a lack of incentives for drug companies, leaving these bacteria, as Schaffner calls it, "a real, very vital threat"
Schaffner said the existence of these bacteria puts responsibility on both food producers and people who would use antibiotics to be more prudent. But he also said the problem could be compounded by the fact that there is currently little financial incentive for drug companies to devise new antibiotics that could fight the threat.
"Clearly, pharma sees the development of new antibiotics to help us treat these drug-resistant infections as high-risk and low-profit," said Schaffner. "I can think of no new product in any line of industry that, once it's released, the experts in that area say, 'Don't use it,' and that's the circumstance when any new antibiotic is created."
MRSA is not untreatable, but when using the stronger antibiotics for it, "You get yourself into a very restricted corner," Schaffner said. "You get patients to whom these drugs are incredibly toxic or you may need to keep patients in the hospital rather than send them home."
Hormone Replacement Therapy
At the start of the decade, millions of women were using hormone replacement therapy to relieve unpleasant symptoms of menopause. It was also used prevent osteoporosis and bone fractures, and heart disease as well.
But as early as 2000, some doctors were recommending against the treatment because of a study in the Journal of the American Medical Association suggesting that it increased the risk of breast cancer.
That alarm greatly increased in 2002 when researchers cut short the Women's Health Initiative study of the treatment, citing concerns over heart problems and strokes in women in the study who received HRT.
Some researchers supported the decision to stop the study, leading many women to stop their HRT, but others felt that it discouraged women who should continue the treatments. Meanwhile, studies have come out with contradictory findings, further confusing women who were unsure what to do. Controversy over HRT continued in 2008, when the International Menopause Society released new guidelines saying that HRT was effective for post-menopausal symptoms and should be considered by women and their physicians.
Following the release of the new guidelines, ABC News contributor Dr. Marie Savard wrote a column for this site in which she tried to clear up some of the confusion.
"There is no question that for a woman with severe hot flashes, sleep disturbance and an annoyingly dry vagina, nothing else works as well as estrogen," Savard wrote. "But the risks of breast cancer, stroke and blood clots from estrogen are hard to ignore... So once again, women are asked to balance the benefits of hormones with the risks and make the best decision for them."
After the collapse of the World Trade Center in 2001, five people died after inhaling anthrax bacteria sent through the mail.
But the public at large had little to fear from a tainted envelope.
"That was obviously not a major health problem but a significant problem for a small number of people who have been getting exposed," said Schwartz. "I think the primary concern was this might be being used to kill some people."
Senators Tom Daschle of South Dakota and Patrick Leahy of Vermont, among other public figures, had letters mailed to their offices containing anthrax.
No one was ever convicted of sending the letters, but the Federal Bureau of Investigation reportedly planned to charge government researcher Bruce Ivins in the case before he committed suicide in the summer of 2008.
As cell phones became more popular this past decade, concerns over the radiation they emit -- and what effect they might have on human health -- have proliferated. Some have worried that their use may be linked to the development of brain tumors.
Thus far, however, most research suggests there is little to worry about.
Animal studies have shown that magnetic fields can affect melatonin levels, so while radiation only shows up in low levels, it's unclear what effect it has on humans. And a Scandinavian study released last week in the Journal of the National Cancer Institute confirmed what many have been saying about cell phone safety, showing no increase in brain cancer among cell phone users.
"Whether they do something worth worrying about, that's another question," said Schwartz. Similar questions are raised about high-voltage power lines, but Schwartz urged calm. "It seems to be an issue where it hasn't completely resolved, but I would say the evidence is that if something is going on it's not that big."
Of course, cell phones present an unquestioned safety hazard to Americans, but not for reasons related to radiation. Studies have shown that their use while driving poses a very real hazard.
"That's pretty clear -- talking on a cell phone and driving is like driving drunk," said Schwartz. "The radiation effects -- that doesn't look like that's a major public health issue. That doesn't look very compelling."