Human viruses cause a variety of maladies, depending on the virus type and the tissues infected. All humans contract multiple viruses throughout the course of life. The severity of viral illnesses ranges from mild to lethal. Cold, flu, stomach and hepatitis viruses are among the most common types of human viruses encountered worldwide.
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The common cold remains a vexing and ubiquitous human affliction. In the United States alone, approximately 1 billion head colds occur each year. Viral infection of the lining of the nasal passages leads to runny nose, watery eyes, scratchy throat and sneezing. The typical course of the common cold is 7 to 14 days. The Centers for Disease Control and Prevention asserts that more than 200 known virus strains can cause a head cold, with rhinoviruses proving the most frequent culprits. Other virus types that may cause a cold include adenovirus, coronavirus, coxsackievirus, echovirus, enterovirus, orthomyxovirus, paramyxovirus and respiratory syncytial virus.
Three types of viruses cause influenza, more commonly known as the flu. Influenza virus types A and B cause seasonal flu infections, which typically occur from late fall through early spring. Influenza type C infections occur far less frequently and typically cause a mild form of the illness. Common symptoms of the flu include body aches, fever, fatigue, headache, scratchy throat, dry cough and a stuffy nose. Influenza immunizations provide protection against infection due to influenza types A and B.
Stomach and Intestinal Viruses
Digestive system viruses invade the tissues of the stomach and upper intestine, causing viral gastroenteritis. Common symptoms include abdominal pain or cramps, diarrhea, nausea and vomiting. Rotavirus frequently causes gastroenteritis in infants and young children. Symptoms include fever, vomiting and watery diarrhea. Norovirus is another common stomach virus that affects both children and adults. Diarrheal symptoms predominate in children with norovirus, whereas adults often experience persistent vomiting. Other common stomach viruses include strains of adenovirus, sapovirus and astrovirus.
The hepatitis viruses infect the liver, causing inflammation. Five different virus cause hepatitis, denoted by the letters A through E. Hepatitis viruses A, B and C predominate in the United States. Hepatitis A virus is contracted through ingestion of stool-contaminated food or water. The virus causes a short-lived episode of hepatitis. Hepatitis B virus can cause acute or chronic liver infection. The virus is present in blood and semen. Sexual intercourse, sharing injection drug equipment and mother-to-child transmission during pregnancy and childbirth are the primary mechanisms of contracting hepatitis B. Hepatitis C virus infection occurs through contact with infected blood. Sharing injection drug equipment is the most frequent route of transmission. Mother-to-child infections also occur. Infection with the hepatitis C virus is usually chronic, but treatment clears the infection in many cases.
Other Human Viruses
A complete list of human viruses would encompass volumes and likely never be complete as new types continue to be discovered. Some viruses are uncommon but greatly concerning because of their potential lethality, such as Ebola and rabies viruses. Others are common and cause a range of human illnesess, such as those in the herpesvirus family, which are responsible for cold sores, genital herpes, infectious mononucleosis, chickenpox, shingles and a number of other illnesses. Human papillomaviruses cause common skin warts but are also a leading cause of cervical cancer. Newer human viruses -- such as HIV, SARS and MERS-CoV -- are always a concern as effective treatment typically lags behind their discovery by many years.