Good bacteria are even present in—and good for—infants. In fact, recent studies show that a young baby may not develop a healthy mix of “good” intestinal bacteria if they are delivered by Caesarean section or don’t drink breastmilk. The scientists believe certain microbes in the gut are linked to a healthy digestive tract and also help to stimulate the immune system. These microbes apparently signal the immune cells to not overreact to certain bacteria and react to others. And if the bacteria are not present, the immune system seems to overreact to something benign, say, dust or food—developing certain medical problems, such as asthma and allergies to foods.

West Nile Virus (an arbovirus) is most commonly transmitted by infected mosquitoes; the insects become infected with the virus when they feed on infected birds, eventually spreading the virus to humans and other animals. It was first detected in North America in 1999 and has since spread across the continental United States and Canada. West Nile Virus has an incubation period of about two to fourteen days, but most people— around 70 to 80 percent—do not develop symptoms. Others who are infected can develop a fever, along with a headache, body aches, joint pains, vomiting, diarrhea, or a rash. Most people recover completely, but a feeling of fatigue and weakness can last for weeks or months. And finally, less than 1 percent of people develop a serious neurologic illness, such as encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). As of this writing, no treatment or vaccine exists for the virus.