A virus is not an independently living organism. Viruses are very small in size, so small that they can only be seen with an electron microscope. They contain mostly only genetic material, either DNA or RNA. A virus has to enter a cell to survive and reproduce itself. Viral infections cause a variety of diseases. Listed below are some of the most common viruses and their properties.

Agent	Disease	Symptoms	Transmission	Incubation Period
Varicella - Zoster Virus	Chickenpox	Itchy rash, which then forms blisters that dry and become scabs in 4-5 days.	Skin to skin or respiratory contact.	2 - 3 weeks
Varicella - Zoster Virus	Shingles	Starts with pain, fever, malaise, chills, gastrointestinal distress, and/or headache , within 5 days, develop swelling or redness of the skin and clusters of clear vesicles, which soon develop into blisters.	Skin to skin or respiratory contact.	13 – 17 days
Hepatitis B Virus	Hepatitis B	Eyes or skin may turn yellow, Loss of appetite, Nausea. Vomiting, fever, stomach or joint pain, tiredness and this can lead to lifelong infection, cirrhosis (scarring) of the liver, liver cancer, liver failure, and death.	Body fluids of an infected person ( blood to blood or sexual contact or sharing needles.  Mother to newborn	7 – 26 weeks
Paramyxovirus	Measles	Symptoms start with cold, fever, cough, conjunctivitis (red eye) and tiredness. After about 3 days, a red blotchy rash starts on the face. It spreads to the rest of the body.	Respiratory contact with nasal or throat secretions.	10 – 14 days
HIV (human immunodeficiency virus)	AIDS  (acquired immune deficiency syndrome)	Loss of immunity to fight infections, rapid weight loss, dry cough, recurring fever or profuse night sweats, fatigue, swollen lymph glands in the armpits, groin, or neck, diarrhea lasting more than a week, white spots or unusual blemishes on the tongue, in the mouth, or in the throat, pneumonia, red, brown, pink, or purplish blotches on or under the skin or inside the mouth, nose, or eyelids, memory loss, depression, and other neurological disorders.	Body fluids of an infected person ( blood to blood or sexual contact or sharing needles.  Mother to newborn	2 months – 10 years
Influenza Virus	Influenza	Typical  illness includes fever and respiratory symptoms, such as cough, sore throat, runny or stuffy nose, as well as headache, muscle aches, and extreme fatigue.	Respiratory droplets or direct contact with mucus	1 – 3 days

Common Properties | Bacteria | Virus | Protozoa | Fungi