The most common contagious diseases are categorized as bacterial, viral, fungal and protozoan. Bacteria are tiny organisms that are pathogenic (cause infectious diseases) or beneficial (aid in digestion). Viral diseases originate from a virus, a sub-microscopic agent that requires a host cell such as a human body for survival. Fungal infections affect the skin, mucous membranes and internal organs. Protozoan infections are from parasites.
The Centers for Disease Control and Prevention reports the loss of 22 million school days yearly in the United States because of the common cold. Viral agents that produce the common cold include the 110 rhinoviruses, parainfluenza, adenoviruses, echovirusus and respiratory syncytial virus. The common cold is highly contagious by inhalation of expelled mucous cold germs in the air and by transferring the viral germs from any surface to the eyes or nose.
Influenza viruses cause highly contagious respiratory illnesses known as the flu. The CDC reports that as many as 20 percent of the people in the United States have the flu every year. Of these, about 36,000 die. Symptoms for respiratory influenza include fever, fatigue, cough, runny nose and aching muscles. Stomach symptoms may include nausea, vomiting and diarrhea. The flu spreads by contact and through coughing and sneezing. Influenza is contagious before symptoms develop and up to seven days after becoming ill.
Strep throat is a highly contagious disease caused by the Group A Streptococcus bacteria. The National Institutes of Health reports strep throat as the most common bacterial throat infection. Symptoms that include sore throat, nausea, fever and chills generally appear between two and five days of exposure. The prognosis with antibiotic treatment is good. Lack of antibiotic intervention can lead to complications such as rheumatic or scarlet fever, ear infection and kidney disease.
Ringworm, a contagious fungal skin infection, presents as ring-shaped patches. Tinea capitis, or scalp ringworm, is a highly contagious form of ringworm seen mostly in children. Scalp ringworm presents as an itchy, pink rash or an area of hair loss with no rash. Body ringworm--tinea corporis--appears on the face, arms, legs and body as round, pink patches with clear centers. Close body contact provides a medium for spreading from person to person.
The Mayo Clinic reports giardiasis, a highly contagious intestinal parasitic infection, as one of the most common waterborne disease in America. Modes of transmission include infested swimming pools, spas and person-to-person contact. Expect treatment by drugs such as metronidazole or tinidazole when symptoms appear.