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Infectious diseases or communicable diseases are diseases caused by a biological agent such as by a virus, bacterium or parasite. They are the invasion of a host organism by a foreign replicator, generally microorganisms, often called microbes, that are invisible to the naked eye. Microbes that cause illness are also known as pathogens. The most common pathogens are various bacteria and viruses, though a number of other microorganisms, including some kinds of fungi and protozoa, also cause disease.
From Airborne Bacteria
You can transmit bacteria to another person through the air by coughing or sneezing. These are common ways to get the bacteria that cause tuberculosis (TB). International airplane travel can expose you to germs not common in your own country.
From Contact with an Infected 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. Some bacterial infections, such as gonorrhea, can be transmitted directly during sexual intercourse.
From Contact with Infectious Material
A common way for some bacteria 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. Day care workers, for example, can pass diarrhea-causing infections from one baby to the next between diaper changes and other childcare practices. It also is possible to transmit infections by shaking someone’s hand or from touching contaminated surfaces, such as a handrail or telephone.
From Contact with a Person who is a Carrier of the Disease
Salmonella bacteria. Image courtesy of Rocky Mountain Laboratories, NIAID, NIH
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.
From Your Household Pet
You can catch a variety of germs from animals, especially household pets. 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 blood infections and meningitis. Meningitis is the 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 give Salmonella bacteria to their unsuspecting owners.
Western black-legged ticks. Image courtesy of CA Department of Health Services.
From an Insect or Other Animal Carrying the Infectious Bacteria
Insects or other animals that carry and transmit pathogens are called "vectors." For example, 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.
From Food or Water Harbouring Infectious Bacteria
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.
From Transplanted Animal Organs Harbouring Infectious Bacteria
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.