There's good news and bad news for Americans. The good news is that the age-adjusted death rate in the U.S. is lower than it's ever been. What's the bad news? The same diseases still kill too many Americans year after year. But progress is being made. Here are the three deadliest diseases in the U.S. -- and what encouraging developments are at hand.

1. Heart disease
Heart disease causes nearly 600,000 deaths in the U.S. each year. Almost two-thirds of that total results from coronary heart disease, which stems from narrowing and blockage of the arteries by deposits of cholesterol and other substances.

Mortality rates for heart disease have been decreasing relatively steadily since 1980. Availability of medications to help individuals control cholesterol levels has no doubt played a major role in this improvement. Now, a new cholesterol drug that could be one of the most effective ever will be available.

Earlier this month, the U.S. Food and Drug Administration approved Merck's (NYSE:MRK) Liptruzet. The drug actually is a combination of two already-approved cholesterol medications -- Lipitor from Pfizer (NYSE:PFE) and Merck's Zetia. In clinical studies, Liptruzet was found to reduce "bad" cholesterol by as much as 61%, significantly better than either of its two component drugs.

This should be great news for patients who have trouble keeping cholesterol levels in check taking one of the currently available medications. And with improvements like this in cholesterol drugs and in other areas, those high numbers of deaths related to heart disease will hopefully keep on falling.

2. Cancer
Cancer continues to be one of the scariest words for too many Americans. Around 575,000 deaths each year across the U.S. result from various forms of cancer.   

Several positives leap out when we look at cancer statistics, though. Cancer-related mortality rates have declined since the early 1990s. While prostate cancer and breast cancer rank at the top of the list when it comes to new diagnoses, many more people now survive battles with these forms of cancer.

Lung cancer, though it ranks third in new cases, stands as the leading cancer-related cause of death and kills more Americans than the next three types of cancer combined. This is discouraging, but there are some breakthroughs making a difference.

Several new drugs are now available that fight lung cancer, including Xalkori from Pfizer. Clinical studies for Xalkori showed progression-free survival rates twice as high as those with standard therapies. However, perhaps even better news for the fight against lung cancer comes from technology used in the development of Xalkori and other cancer drugs.

Drugs like Xalkori reached the market in around half the time that most take to do so in part through the use of genetic screening. Next-generation sequencing technology from Illumina (NASDAQ:ILMN) and others now enables researchers to quickly sequence DNA. This speeds up drug development by helping companies target drugs for specific gene mutations and shortening the amount of time required to enroll patients. With more effective treatments reaching the market sooner, more lives could potentially be saved.

3. Chronic lower respiratory disease
Around 140,000 Americans die each year from chronic lower respiratory disease, primarily due to chronic obstructive pulmonary disease, or COPD. Smoking is the key risk factor for COPD, causing as much as 90% of COPD deaths.

Unfortunately, chronic lower respiratory disease stands out as the only one of the top three deadliest diseases in the U.S. that saw an increase in age-related death rates in the most recent CDC data. However, cigarette smoking trends continue to drop for both adults and students. This could portend a better outlook in the future in the battle against this disease.

Another promising sign comes from the development of effective treatments for COPD. GlaxoSmithKline (NYSE:GSK) and Theravance (NASDAQ:THRX) obtained FDA approval just days ago for a new COPD drug called Breo. Even more hope hinges on another drug potentially on the way from the two companies -- Anoro. The FDA set a decision date of Dec. 18 for the drug. If approved, Anoro would become the first treatment to combine a long-acting muscarinic antagonist and a long-acting beta-2 agonist.

Doing good
Not only are all of the companies mentioned doing good by helping fight America's deadliest diseases, they're also doing well for investors. Theravance and Illumina rank as the best-performing stocks of the group, with one-year gains of 90% and 58%, respectively. Pfizer shares jumped more than 30% in the past year, with Merck's stock rising by 22%. Glaxo lagged behind, but still recorded a respectable 15% gain during the period.

There are many other companies, universities, and government researchers who are also hard at work to find new ideas for tackling these challenging diseases. Hopefully, we will soon reach the point where the good news outweighs the bad.

Fool contributor Keith Speights has no position in any stocks mentioned. The Motley Fool recommends Illumina. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.