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According to healthcare experts, infectious diseases caused by microbes are responsible for more deaths worldwide than any other single cause. They estimate the annual cost of medical care for treating infectious diseases in the United States alone is about $120 billion.

The science of microbiology explores how microbes work and how to control them. It seeks ways to use that knowledge to prevent and treat the diseases microbes cause. The 20th century saw an extraordinary increase in our knowledge about microbes. Microbiologists and other researchers had many successes in learning how microbes cause certain infectious diseases and how to combat those microbes.

Unfortunately, microbes are much better at adapting to new environments than are people. Having existed on Earth for billions of years, microbes are constantly challenging human newcomers with ingenious new survival tactics.

  • Many microbes are developing new properties to resist drug treatments that once effectively destroyed them. Drug resistance has become a serious problem worldwide.
  • Changes in the environment have put certain human populations in contact with newly identified microbes that cause diseases we have never seen before, or that previously occurred only in isolated populations.
  • Newly emerging diseases are a growing global health concern. Since 1976, scientists have identified approximately 30 new pathogens, such as HIV.

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Some Ways Microbes Are Transmitted

Some microbes can travel through the air

You can transmit microbes to another person through the air by coughing or sneezing. These are common ways to get viruses that cause colds or flu, or the bacteria that cause tuberculosis. Interestingly, international airplane travel can expose you to germs not common in your own country.

Close contact can pass germs to another person

Scientists have identified more than 500 types of bacteria that live in our mouths. Some keep the oral environment healthy, while others cause problems like gum disease. One way you can transmit oral bacteria is by kissing.

Microbes such as HIV, herpes simplex virus type 2, which causes genital herpes, and Neisseria gonorrhoeae bacteria, which causes gonorrhea, are examples of germs that cyou can pass to another during sexual intercourse.

You can pick up and spread germs by touching infectious material

A common way for some microbes to enter the body, especially when caring for young children, is through unintentionally passing feces from hand to mouth or the mouths of young children. Infant diarrhea is often spread in this way. Daycare workers, for example, can pass diarrhea-causing rotavirus or Giardia lamblia (protozoa) from one baby to the next between diaper changes and other childcare practices.

It also is possible to pick up cold viruses from shaking someone’s hand or from touching contaminated surfaces, such as a doorknob or computer keyboard.

A healthy person can carry germs and pass them onto others

The story of “Typhoid Mary” is a famous example from medical history about how a person can pass germs on to others, yet not be affected by those germs. The germs in this case were Salmonella typhi bacteria, which cause typhoid fever and are usually spread through food or water.

In the early 20th century, Mary Mallon, an Irish immigrant, worked as a cook for several New York City families. More than half of the first family she worked for came down with typhoid fever. Through a clever deduction, a researcher determined that the disease was caused by the family cook. He concluded that although Mary had no symptoms of the disease, she probably had had a mild typhoid infection sometime in the past. Though not sick, she still carried the Salmonella bacteria and was able to spread them to others through the food she prepared.

Germs from your household pet can make you sick

You can catch a variety of germs from animals, especially household pets. The rabies virus, which can infect cats and dogs, is one of the most serious and deadly of these microbes. Fortunately, rabies vaccine prevents animals from getting rabies. Vaccines also protect people from accidentally getting the virus from an animal. In addition, vaccines prevent people who already have been exposed to the virus, such as through an animal bite, from getting sick.

Dog and cat saliva can contain any of more than 100 different germs that can make you sick. Pasteurella bacteria, the most common, can be transmitted through bites that break the skin causing serious, and sometimes fatal, diseases such as meningitis. Meningitis is inflammation of the lining of the brain and spinal cord.

Warm-blooded animals are not the only ones that can cause you harm. Pet reptiles such as turtles, snakes, and iguanas can giveSalmonella bacteria to their unsuspecting owners.

You can get microbes from tiny critters

Mosquitoes may be the most common carriers, also called vectors, of pathogens. Anopheles Plasmodium, which causes malaria, from the blood of an infected person and transmit the protozoan to an uninfected person.

Fleas that pick up Yersinia pestis bacteria from rodents can then transmit plague to humans.

Ticks, which are more closely related to crabs than to insects, are another common vector. The tiny deer tick can infect humans with Borrelia burgdorferi, the bacterium that causes Lyme disease, which the tick picks up from mice.

Some microbes in food or water could make you sick

Every year, millions of people worldwide become ill from eating contaminated foods. Although many cases of foodborne illness or “food poisoning” are not reported, the Centers for Disease Control and Prevention (CDC) estimates there are 76 million cases of such illnesses in the United States each year. In addition, CDC estimates 325,000 hospitalizations and 5,000 deaths are related to foodborne diseases each year. Microbes can cause these illnesses, some of which can be fatal if not treated properly.

Poor manufacturing processes or poor food preparation can allow microbes to grow in food and subsequently infect you. Escherichia coli (E. coli) bacteria sometimes persist in food products such as undercooked hamburger meat and unpasteurized fruit juice. These bacteria can have deadly consequences in vulnerable people, especially children and the elderly.

Cryptosporidia are bacteria found in human and animal feces. These bacteria can get into lake, river, and ocean water from sewage spills, animal waste, and water runoff. Millions can be released from infectious fecal matter. People who drink, swim in, or play in infected water can get sick.

People, including babies, with diarrhea caused by Cryptosporidia or other diarrhea-causing microbes such as Giardia and Salmonella, can infect others while using swimming pools, waterparks, hot tubs, and spas.

Transplanted animal organs may harbor germs

Researchers are investigating the possibility of transplanting animal organs, such as pig hearts, into people. They, however, must guard against the risk that those organs also may transmit microbes that were harmless to the animal into humans, where they may cause disease.

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How You Can Protect Yourself From Microbes

We become immune to germs through natural and artificial means. As long ago as the 5th century B.C., Greek doctors noticed that people who had recovered from the plague would never get it again—they seemed to have become immune or resistant to the germ. You can become immune, or develop immunity, to a microbe in several ways. The first time T cells and B cells in your immune system meet up with an antigen, such as a virus or bacterium, they prepare the immune system to destroy the antigen.

Naturally acquired immunity

Because the immune system often can remember its enemies, those cells become active if they meet that particular antigen again. This is called naturally acquired immunity.

Another example of naturally acquired immunity occurs when a pregnant woman passes antibodies to her unborn baby. Babies are born with weak immune responses, but they are protected from some diseases for their first few months of life by antibodies received from their mothers before birth. Babies who are nursed also receive antibodies from breast milk that help protect their digestive tracts.

Artificial immunity

Artificial immunity can come from vaccines. Immunization with vaccines is a safe way to get protection from germs. Some vaccines contain microorganisms or parts of microorganisms that have been weakened or killed. If you get this type of vaccine, those microorganisms (or their parts) will start your body’s immune response, which will demolish the foreign invader but not make you sick. This is a type of artificially acquired immunity.

Immunity can be strong or weak and short- or long-lived, depending on the type of antigen, the amount of antigen, and the route by which it enters your body. When faced with the same antigen, some people’s immune system will respond forcefully, others feebly, and some not at all.

The genes you inherit also can influence your likelihood of getting a disease. In simple terms, the genes you get from your parents can influence how your body reacts to certain microbes.

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Last Updated July 25, 2014

Last Reviewed July 25, 2014