Over millions of years, microbes and humans have formed a unique relationship. The human immune system developed in response to various threats by bacteria, viruses and other microbes. This allowed humans to walk among the microbes that lived in the soil, water and air without constant infections and illnesses. The human body, however, did not eliminate all of the microbes. Instead, it became a home for trillions of microbes that still thrive in our bodies and on our skin.
Even with the largely beneficial relationship between humans and microbes, diseases caused by microbes are still common. Many infectious diseases result from microbes living in the environment, and even the ones found on and in the human body. Scientists study microbes to understand how they function, and to learn how they cause diseases. They work with public health officials, lawmakers and others to keep the public safe from microbial diseases. Scientists also use microbes to their advantage, developing new drugs and treatments for a wide range of non-microbial diseases.
Infectious Diseases & Microbes
Throughout history, humans have been exposed to a wide number of infectious diseases, such as plague, typhoid fever, malaria, influenza and AIDS. It wasn’t until the late nineteenth century, though, that scientists clearly understood that diseases such as these were caused by microbes. Armed with this knowledge, they developed vaccines, antibiotics and other methods to treat and prevent infectious diseases. Medical treatments, however, were not the first protection against microbes. Long before the antibiotic penicillin was used against bacterial infections, the human immune system had been fighting dangerous microbes.
Infectious diseases can spread in many ways. Bodily fluids — such as from a sneeze — can contaminate surfaces with microbes, such as the viruses that cause the common cold and flu. Other microbes pass between people during sexual contact. The virus that causes AIDS is transmitted in this fashion. Infectious diseases — such as cholera — can also be spread through contaminated food, water or air. Even insects can transmit microbial diseases to humans. Malaria is transferred to humans by mosquitoes. When a mosquito carrying a disease-causing protist bites a person, the protist enters the bloodstream.
Digestive System & Microbes
Scientists estimate that 80 kinds of microbes live in the mouth, and another 80 live in the rest of the digestive system. Some of these microbes help break down food particles, while others provide vitamin K and other nutrients. Recen research on mice suggests that bacteria living in the intestines may also affect mood. During the study, mice that were fed certain bacteria were less anxious, and also showed changes in the levels of stress hormones.
Not all microbes in the digestive system are beneficial. Bacteria, viruses and fungi can all cause intestinal infections. These illnesses are often accompanied by diarrhea and inflammation of the stomach and intestines. Infection by harmful bacteria can occur in several ways. Contaminated shellfish, undercooked meat and unpasteurized dairy products can all contain harmful microbes. They may also be picked up from contaminated surfaces, such as countertops or door handles. Bodies of water sometimes harbor harmful microbes that are picked up while bathing or swimming.
Microbes on the Skin
Human skin is covered in microbes. On average, 10 million bacteria live on a square centimeter of skin. The number varies on different parts of the body. Oily skin on the side of the nose or in a sweaty armpit can harbor ten times as many. That number increases to 1,000 inside the nose and throat. Some locations, however, are devoid of microbes. This includes the bladder and deeper parts of the lungs, both of which are essentially open to the outside world.
The extensive community of microbes living on the skin forms a barrier against infection by more harmful microbes. When the skin is broken, either by injury or during surgery, there is an increased risk of infection by bacteria, viruses or fungi. Bacteria can cause skin lesions, inflammation or boils. Infection of the skin by a virus can result in warts, as is the case with Human Papilloma Virus (HPV) and plantar warts. Athlete’s foot and jock itch are two examples of fungal infections of the skin.
Public Health and Sanitation & Microbes
Public health and sanitation barely existed during the American Civil War in the mid-nineteenth century. This resulted in outbreaks of infectious diseases such as typhoid fever, cholera and dysentery. These epidemics were especially common in urban areas, where many people shared water sources and created waste. After Louis Pasteur and other scientists proved the connection between microbes and disease, public health officials looked to science for guidance. They designed new practices and laws to protect the public water supply, deal with human waste and handle outbreaks of disease.
In the late 1800‘s, filtration of municipal water became a standard practice. This reduced the number of disease-causing microbes in water supplies. Later, chlorination further protected the public from infectious diseases caused by water-borne microbes. In hospitals, the sterilization of medical equipment with steam reduced the spread of infectious diseases to patients. Restaurants and cafeterias are now required to follow sanitary procedures to prevent contamination of food. In addition, most communities require the vaccination of children at certain ages. This has reduced the occurrence of infectious diseases such as polio and measles.
Medical Applications of Microbes
As scientists learned how microbes cause diseases, they developed methods for treating and preventing them. Vaccines provide people with protection against disease-causing microbes — often viruses. During vaccination, doctors inject weakened or dead microbes — or pieces of the microbe — into the bloodstream to stimulate the immune system without causing the disease. In 1928, the age of antibiotics began with the discovery of the drug penicillin. This compound was isolated from a fungus, and enabled doctors to treat bacterial infections without harsh chemicals.
In addition to vaccines and antibiotics, microbes have been used to create other medical and pharmaceutical drugs. Some compounds, such as penicillin, are naturally occurring compounds. Microbes can also be altered genetically so that they produce drugs instead of — or in addition to — their usual products. Insulin — the drug used to treat diabetes — can now be produced at a lower cost by using a common intestinal bacterium. Other examples of drugs derived from microbes include compounds used to treat cystic fibrosis, hemophilia and cancer.