Story highlights An investigation of an anthrax incident at the CDC concludes no lab workers were exposed

The investigation turned up another safety issue involving bird flu

The CDC director is "upset" and "angry" that it happened

Concluding its investigation into the unintended anthrax exposure at the Centers for Disease Control and Prevention in Atlanta, the CDC says it has found another more "distressing" problem due to lab workers not following protocol.

The CDC held a news conference to discuss the conclusion of its investigation Friday. It determined that while it is "not impossible" that the staff was exposed to viable B. anthracis (anthrax), it is "extremely unlikely" that this happened.

The lab workers' health is being monitored, and they were given antibiotics as a precaution. No one who may have been exposed has gotten sick.

The CDC report says the potential exposure happened between June 6 and June 13. A lab that had been preparing the anthrax samples for use in two other labs on the CDC's Atlanta campus "may not have adequately inactivated the samples."

Photos: They discovered the cure for ... Photos: They discovered the cure for ... They discovered the cure for ... – Before the term "vaccination" was coined, millions died every year from infectious diseases such as smallpox, tuberculosis and yellow fever. But the birth of immunology brought with it cures for some of the world's most deadly diseases. Click through the gallery to see who is credited with developing these life-saving vaccines: Hide Caption 1 of 17 Photos: They discovered the cure for ... Hide Caption 2 of 17 Photos: They discovered the cure for ... Dr. Edward Jenner – Dr. Edward Jenner is known as the founder of immunology . He first attempted vaccination against smallpox in 1796 by taking cowpox lesions from a dairymaid's hands and inoculating an 8-year-old boy. On May 8, 1980, the World Health Assembly announced that smallpox had been eradicated across the globe. Samples of the virus are still kept in government laboratories for research as some fear smallpox could one day be used as a bioterrorism agent Hide Caption 3 of 17 Photos: They discovered the cure for ... Hide Caption 4 of 17 Photos: They discovered the cure for ... Louis Pasteur – You probably know Louis Pasteur as the man who invented pasteurization. But Pasteur also developed the first vaccines for rabies and anthrax. The French microbiologist grew rabies in rabbits first to weaken the virus. Then in 1885, he injected the vaccine into a 9-year-old boy who had been attacked by a dog; it was a success and Pasteur became famous. Hide Caption 5 of 17 Photos: They discovered the cure for ... Hide Caption 6 of 17 Photos: They discovered the cure for ... Gaston Ramon – Veterinarian Gaston Ramon used a formaldehyde solution to deactivate the toxic part of the diphtheria toxin, which allowed scientists to later inject inactive diphtheria into humans as a vaccine. Ramon's discovery helped lead researcher P. Descombey to develop a similar toxoid for tetanus in 1924; it was first used in soldiers during World War II. Hide Caption 7 of 17 Photos: They discovered the cure for ... Hide Caption 8 of 17 Photos: They discovered the cure for ... Jules Bordet – In 1900, Belgian physician Jules Bordet worked with Octave Gengou to isolate the microbe that causes bordetella pertussis, the pathogen that leads to whooping cough. The isolated bacterium was used to develop the pertussis vaccine. Bordet won the 1919 Nobel Prize in Medicine for his work in immunology. Hide Caption 9 of 17 Photos: They discovered the cure for ... Hide Caption 10 of 17 Photos: They discovered the cure for ... Camille Guerin and Albert Calmette – Camille Guerin, left, and Albert Calmette developed the Bacille Calmette-Guerin vaccine for tuberculosis disease. The vaccine was first used in humans in 1921, after 13 years of animal testing. Hide Caption 11 of 17 Photos: They discovered the cure for ... Hide Caption 12 of 17 Photos: They discovered the cure for ... Maurice Hilleman – The CDC recommends 14 vaccines for American children younger than 6. Of those 14, microbiologist Maurice Hilleman developed eight, including the MMR vaccination and the FDA-approved vaccines for chickenpox, hepatitis A, hepatitis B, meningitis and pneumonia. After his death in 2005, Merck pharmaceuticals dedicated The Maurice R. Hilleman Center for Vaccine Manufacturing in Durham, North Carolina, to its most successful researcher. Hide Caption 13 of 17 Photos: They discovered the cure for ... Hide Caption 14 of 17 Photos: They discovered the cure for ... Max Theiler – Although many scientists contributed to the development of the current vaccine for yellow fever, Max Theiler was awarded a Nobel Prize in 1950 for his work. The South African researcher first inoculated mice through their brains, and then took a serum from the protected mice to inject in humans. Hide Caption 15 of 17 Photos: They discovered the cure for ... Hide Caption 16 of 17 Photos: They discovered the cure for ... Dr. Jonas Salk – Dr. Jonas Salk was something of a scientific hero after developing the polio vaccine. Before it was widely used, more than 45,000 Americans contracted the virus each year. By 1962 -- less than 10 years after it was first tested -- the number of cases had dropped to 910, according to the Salk Institute. "Salk never patented the vaccine, nor did he earn any money from his discovery, preferring to see it distributed as widely as possible," his biography on Salk.edu says. Hide Caption 17 of 17

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That means the other labs were working with samples thought to be harmless; therefore, workers didn't wear protective equipment they would otherwise have been using when handling such infectious material.

Procedures in two of the three labs may have exposed workers to the anthrax, the CDC said. Hallway and lab areas have since been decontaminated. The leader of one of the labs was reassigned shortly after the incident was revealed. CDC spokesman Tom Skinner would not confirm the name of the person who was reassigned.

There are three types of anthrax infection: cutaneous (through the skin), inhalation (through the lungs) and gastrointestinal (through digestion).

According to the report, the exposure happened because the lab that was handling the dangerous material initially didn't use an approved sterilization technique. It didn't have a written plan reviewed by senior staff to make sure all safety protocols were followed, and there was a limited knowledge of peer-reviewed literature about the process that would make it less dangerous. The lab also did not have a standard operating procedure that would make sure the transfer of the material would be safe.

Bottom line, "the scientists failed to follow a scientifically derived and reviewed protocol that would have assured the anthrax was deactivated," according to CDC Director Tom Frieden . It "should have happened, and it didn't."

During the investigation "multiple other problems were found," according to Frieden.

Investigators found another troubling case that involved a dangerous transfer of material. This happened six weeks ago. But what's "most distressing," according to Frieden, is that he had found out about it "less than 48 hours ago."

In this case, a culture of nonpathogenic avian influenza, meaning a type of the flu that is not that dangerous, was unintentionally cross-contaminated with a potentially deadly kind of flu -- the highly pathogenic H5N1. This strain has killed millions of birds and infected over 600 people over the last decade.

In the case of this contamination, the CDC says none of the lab workers were exposed to the dangerous virus. It was, however, shipped to a lab run by the United States Department of Agriculture.

The CDC has closed the labs that were involved in the incidents. The lab where the flu contamination occurred won't reopen until the lab puts better safety procedures in place, Frieden says.

The lab involved in the anthrax incident will remain closed for such dangerous microbes until Frieden personally approves its reopening under conditions that prevent any such mishap from happening again, Frieden said.

The CDC has also stopped moving any biological material out of its two highest-level labs while their procedures are being reevaluated.

As a result of these problems, the CDC has created a high-level group of leaders who will work on lab safety issues. It has also started putting together an external advisory group to take a closer look at lab safety. And the CDC is carefully reviewing all the other procedures for labs working with dangerous pathogens, and it has put together a rapid-response command structure to hopefully help the labs avoid future incidents.

"I will say that I'm just astonished that this could have happened here," Frieden said.

He stands by his labs, which are considered some of the best in the world, and he stands by his scientists. But he added that he is "upset," "angry" and has lost sleep over the incidents, and he is "working around the clock" to make sure it never happens again.