The hypodermic needle has delivered lifesaving drugs and vaccines to millions of people worldwide, but the medical marvel also carries deadly baggage: a largely invisible epidemic of diseases spread by unsafe medical use of needles and syringes, mostly in developing countries.

The 16 billion injections administered in developing countries each year cause 21 million cases of hepatitis B, 2 million cases of hepatitis C and 260,000 cases of HIV, the World Health Organization estimates.

The problem, which accounts for more than one-third of all hepatitis B and C infections and 5 percent of HIV cases in those countries, stems from three factors: unnecessary shots given when pills would do; frequent reuse of contaminated needles; and improper disposal, creating piles of needles that can accidentally stick scavengers. Use of illicit drugs also plays a role, but mostly in developed countries, health officials say.

"It is perhaps the single largest man-made medical disaster of the 20th century," said Dr. Robert Chen of the Centers for Disease Control and Prevention. "It's amazing humanity has done so little to solve this problem."

Quietly but with determination, Chen and Georgia Tech mechanical engineer Jonathan Colton are pursuing a possible solution: plastic needles to replace the ubiquitous stainless steel.

Plastic needles could be waved over a lit candle to render reuse impossible, the researchers say. The needles could be recycled and melted to be made into water buckets, eating utensils or even construction bricks, eliminating disposal concerns while also helping provide needed items.

"It's easier to disable a plastic needle than a steel needle, and it's easier to recycle the material," Colton said.

If the engineering challenges of converting pliable plastic into a tiny but sturdy and sharp device can be overcome, the needles also wouldn't increase the wince factor of getting shots. "We think the pain and trauma will be the same as a steel needle," Colton said.

Making needles out of plastic is the latest modification to be considered since the hypodermic needle was invented 150 years ago.

Syringes and needles were both made of metal until the 1890s, when glass syringes hit the scene. The first mass-produced, disposable glass syringe came in 1954, for use with the Salk polio vaccine. (The Sabin polio vaccine used in most countries today is oral.) A single-use plastic syringe soon followed, preventing most contamination problems in wealthier nations, where safety caps and other features in recent years have also helped reduce health worker injuries.

But in developing countries, reusing plastic disposables has become the cheapest route to a steady supply. Some of those syringes and needles are tainted by bits of blood from previous patients that harbor deadly viruses.

The problem of unsafe needles simmered in the 1970s as health officials launched global immunization campaigns. By the 1990s, it boiled over as HIV stretched worldwide and outbreaks of hepatitis, Ebola and other viruses were linked to unsafe injections. Health officials hoping to expand measles and tetanus vaccination throughout the developing world didn't want to do more harm than good.

Many viruses, and some bacteria, can be passed by dirty needles. Health officials became most concerned about hepatitis B and C and HIV, because those viruses can linger in people for many years, often silently, while others using the needles "downstream" are unknowingly placed at risk. Both of the hepatitis viruses can cause liver failure and cancer; medications can keep people with HIV alive for a long time, but the drugs often aren't available in developing countries.

Preferring 'real' medicine

A technological solution arose: "auto-disabled" syringes, equipped with clips that jam the plunger after a single injection or other means of blocking reuse. The WHO, UNICEF and other organizations started demanding such syringes for immunization efforts.

But a major obstacle remains: in developing countries, less than 10 percent of injections are for vaccines. Most are for treatment, and up to half --- an antibiotic injection instead of a pill, for example --- are unnecessary, Chen said. The belief in many places that only shots provide "real" medicine, coupled with doctors' often getting paid more for giving injections than dispensing pills, challenge efforts to reduce the use of needles in routine care.

The use of needles in treatment also complicates disposal efforts: Even if organizers of immunization campaigns carefully collect each of their needles, the chaos of poorly funded treatment clinics leaves many used syringes and needles discarded in disarray.

In 1999, Chen, who works for the CDC's National Immunization Program, took a sabbatical at the WHO in Geneva. He helped found the Safe Injection Global Network, a WHO effort to address the problem.

When Chen returned to Atlanta, he teamed up with Colton to work on their own hopeful strategy: plastic needles, which could prevent unsafe reuse while also reducing the dangers of improper disposal. The relative indestructibility of steel needles suggests that they are "overengineered" for simple procedures such as injections, Chen said.

Last year, the two researchers received a $60,000 CDC-Georgia Tech "seed" grant to develop their idea.

From cars to syringes

Colton, who specializes in new types of plastics and the design of car and ship parts, is wrestling with the mechanics: how to fashion a needle an inch long and not much thicker than a human hair, able to pierce skin and allow liquid to flow through rapidly without bending.

He has used injection molding to create several models. So far, they can poke through rubber gloves and allow cold coffee to easily pass through. The trick is to make a plastic needle thin, sharp and sturdy enough to withstand at least two punctures: one of the rubber stopper on a vial of medication, the other of human skin.

Colton is in discussions with a leading medical device company. The needles would have to show promise in animal and human studies before being marketed.

For now, the researchers are happy to apply their disparate fields of public health and engineering to a common goal: solving a global problem that most people don't even know exists.

"It's nice to work on a very challenging project that has the potential to help people," Colton said.