"If we are not careful, we will soon be in a post-antibiotic era," CDC director Dr. Tom Frieden told reporters this week. "Without urgent action now, more patients will be thrust back to a time before we had effective drugs."

You've likely heard warnings that antibiotic-resistant bacteria and fungi present a major health problem for the world, but the CDC (Centers for Disease Control and Prevention, a US federal agency) has quantified those dangers in a new report this week that, for the first time, ranks the diseases based on the threat they pose.

At least two million in the US are infected with diseases resistant to some types of antibiotics, and those illnesses directly lead to 23,000 deaths per year, according to the new CDC report. The agency stresses that these numbers are underestimates intended to set a universally-agreed baseline of the effects of antibiotic-resistant diseases. Those who are infected with such diseases but didn't die directly due to them aren't included in these numbers, for instance.

The CDC warns that if the US and others do little to change our current course, doctors will not only have limited ways to fight off infectious diseases, but lifesaving modern medical treatments — like chemotherapy, dialysis, complex surgery, organ transplants, and treatment of conditions like diabetes, asthma, and rheumatoid arthritis — will no longer be safe because of the decreased ability to prevent secondary infections.

The 114-page paper, written as a highly-accessible guide to antibiotic-resistant diseases in the US, focuses on 18 pathogens. Three are marked as "urgent threats," meaning each is "an immediate public health threat that requires urgent and aggressive action." One particularly fearsome disease is carbapenem-resistant Enterobacteriaceae (CRE, pictured above), which typically spreads in hospital environments. It can resist even the strongest antibiotics, and the CDC notes that nearly half of all those who get rare bloodstream infections of the disease die. In total, CRE is said to account for a minimum of 9,000 infections and 600 deaths per year in the US.

Four primary methods of limiting antibiotic resistance are highlighted in the report: infection prevention (by immunization and proper sanitization), disease tracking, antibiotic stewardship (i.e. limiting inappropriate use of the drugs), and continued development of new drugs.

The CDC notes that preventing the misuse of antibiotics is likely "the single most important action needed to greatly slow down the development and spread of antibiotic-resistant infections," adding that up to half of all use of the drugs in humans is unnecessary or applied in the wrong doses. It's not just humans however: around 70 to 80 percent of antibiotics produced in the US are used on livestock to not only prevent and treat disease, but also to fatten animals. The report says in no uncertain terms that "the use of antibiotics for promoting growth is not necessary, and the practice should be phased out."

The agency adds that antibiotics "are a limited resource," and that misuse not only limits our ability to fight diseases in the future, but also puts patients at risk to side affects and allergic reactions. Also, since antibiotics kill healthy bacteria in the body, they make patients susceptible to other diseases. For example, Clostridium difficile, one of the other "urgent threats" in the CDC report, causes a minimum of 250,000 infections per year and 14,000 deaths that are closely related to the use (and overuse) of antibiotics. The disease causes life-threatening diarrhea, particularly in older patients.