Antibiotics are considered the keystone of modern medicine, but their excessive use continues to generate unwanted side effects. The U.S. Centers for Disease Control and Prevention (CDC) isn’t historically known for using pessimistic language, but lately they’ve been sounding the alarms about the consequences of overusing antibiotics. CDC director Tom Frieden has warned of “nightmare bacteria,” those that have evolved defenses against modern antibiotics. This leads to to strains that can cause fatal infections. While specialists are making strides to preserve the effectiveness of antibiotics and to slow potential infections through better policy, the overuse of antibiotics continues to have severe health consequences for the U.S. and around the world. Learn About the 10 Worst Disease Outbreaks in U.S. History »

1. Antibiotics Increase Fatal Diarrhea Cases in Children Because the majority of common colds are viral, using antibiotics to treat them does nothing to stop the infection and can create unwanted side effects. Still, studies have shown that half of antibiotics prescribed for children are for upper respiratory infections associated with the common cold. A new CDC study shows that children given antibiotics for routine upper respiratory infections are more susceptible to aggressive antibiotic-resistant strains of the bacteria commonly known as C. diff. The study found that 71 percent children who suffered C. diff infections had been given courses of antibiotics for respiratory, ear, and nose illnesses 12 weeks before infection. “When antibiotics are prescribed incorrectly, our children are needlessly put at risk for health problems including C. difficile infection and dangerous antibiotic-resistant infections,” Frieden said in a recent statement. C. diff, a bacteria found in the human gut, can cause severe diarrhea and is responsible for 250,000 infections in hospitalized patients and 14,000 deaths every year among children and adults. A recent study published in the journal Pediatrics shows that antibiotic prescriptions for children have plateaued, but continued attention is needed to reduce the amount of broad-spectrum antibiotics given to children for certain conditions. Read More About the CDC’s Effort to Rein In Antibiotic Prescription Practices »

2. Antibiotics Can Upset Sensitive Gut Flora Your intestines contain around 100 trillion bacteria of various strains. While some can be deadly, there’s a natural balance in the gut that can be thrown out of whack by antibiotics. These helpful bacteria, known as gut flora, support immunity and proper digestion. Aggressive antibiotics, while helpful if you have a serious infection, can wipe out many good gut bacteria while leaving those immune to antibiotics to flourish. That’s the case with C. diff diarrheal infections. Many people, especially children, are vulnerable to unwelcome side effects of unnecessary antibiotics, including lasting changes to their gut flora. Learn 6 Surprising Facts About Your Gut Bacteria »

3. Antibiotics Help Teach Good Bacteria to Go Bad Bacteria have evolved defenses against antibiotics through the process of horizontal gene transfer. Essentially, bacteria don’t need to reproduce to pass along their genetic protection from antibiotics. They can simply pass along these genes to fellow bacteria like students passing notes in a classroom. One study found that bacteria passing through the colon can transfer their resistance genes to other forms of bacteria. A study from Birkbeck, University of London, released this week shows how bacteria secrete genes between membranes, possibly giving scientists a clue about how to stop the gene transfer. Read More: Aggressive Antibiotics Lead to Stronger Bacteria »

4. Antibiotics Are Increasing Cases of Untreatable Gonorrhea Along with C. diff, the CDC is aggressively tracking cases of antibiotic-resistant gonorrhea. This untreatable gonorrhea not only causes pain but also has been linked to pelvic inflammatory disease, ectopic pregnancy, tubal infertility, and neonatal eye infections, among other conditions. A specific strain, Neisseria gonorrhoeae, has developed resistance to the antibiotics typically used to treat these infections. Currently, cephalosporin antibiotics are the only class that meets the CDC’s standards to fight resistant gonorrhea. The emerging threat has experts concerned, and it indicates that a better understanding of the bacteria’s epidemiology is needed. In 2012, 334,826 cases of gonorrhea were reported in the U.S., and the majority of new infections occurred in people ages 15 to 24, according to the CDC . Learn How to Spot Gonorrhea and Other STDs »