(Norene Anderson/ Livestrong.com) — 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.
Common Cold
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
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. (…)