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Viruses are very tiny organisms with their diameters in nanometers level. Viruses exist almost everywhere, such like in the air, soil and animal body. Human body also habors a lot of viruses, but most of them do not cause diseases. Human immune system can generate antibodies against viruses, the antibody will recognize the virus and destroy it.

Sometimes the virus and human immune system can establish a balance. However when the human immune system is weak due to various reasons, the viruses can break the balance and cause problems, became so called opportunist pathogen. For example, the BK polyomavirus is presented in the kidney of ~80% population, however it only cause nephritis in limited population. But that don't mean all viruses are not harmful, or opportunist pathogens, some viruses can cause serious diseases, including AIDS, SARS, Hepatitis, Herpes Simplex etc.

List of diseases caused by viruses:

Disease Name	Virus Type	Organs Affected	Transmission
• Influenza	RNA	Respiratory Tract	Droplets
• Adenovirus Infections	DNA	Lungs, Eyes	Droplets, Contact Droplets
• Respiratory Syncytial Disease	RNA	Respiratory Tract	Droplets
• Rhinovirus Infections	RNA	Upper Respiratory Tract	Droplets,Contact
• Herpes Simplex	DNA	Skin,Pharynx, Genital organs	Contact
• Chicken pox ( Varicella)	DNA	Skin, Nervous System	Droplets, Contact
• Measles (Rubeola)	RNA	Respiratory Tract, Skin	Droplets, Contact
• German Measles ( Rubella)	RNA	Skin	Droplets, Contact
• Mumps (Epidemic Parotitis)	RNA	Salivary Glands, Blood	Droplets
• Small Pox (Variola)	DNA	Skin, Blood	Contact, Droplets
• Warts Kawasaki Disease	DNA	Skin	?
• Yellow Fever	RNA	Liver, Blood	Mosquito ( Aedes Aegypti)
• Dengue Fever	RNA	Blood, Muscles	Mosquito ( Aedes Aegypti )
• Hepatitis A	RNA	Liver	Food, Water, Contact
• Hepatitis B	DNA	Liver	Contact with body Fluids
• NANB Hepatitis	RNA	Liver	Contact with body Fluids
• Viral Gastroenteritis	Many RNA Viruses	Intestine	Food, Water
• Viral Fevers	Many RNA Viruses	Blood	Contact,arthropods
• Cytomegalovirus Disease	DNA	Blood, Lungs	Contact, Congenital transfer
AIDS	Retrovirus ( RNA)	T-lymphocytes	Contact with body Fluids
• Rabies	RNA	Brain, Spinal cord	Conact with body Fluids
• Polio	RNA	Intestine,Brain, Spinal Cord	Food, Water, Contact
• Slow Virus Disease	Prions	Brain	?
• Arboviral Enephalitis	Many RNA viruses	Brain	Anthropods
• Ebola	Ebola viruses	whole body	bodily fluids

Virus has two types, DNA virus and RNA virus. The genome of DNA virus is consisted of DNA, and similarily RNA is the genetic material of a RNA virus. The virus genome is typically several kilo basepairs (kb), and usually just contains several genes. They use the host's system for replication and gene expression. The structure of virus is consisted of the small genetic materials and a protein envelope. Since higher temperature can cause protein denaturation, viruses are sensitive to heat. However, they can remain hibernate in lower temperature for a long time. For example, viruses were found preserved in millions years old ice in Antarctica.

Human immune system usually can recognize a specific piece of a virus protein sequence, thus identify and destroy them. And the virus specific information can be remembered by the immune system, sometimes through the whole life. This mechanism has been used to make vaccines. Usually vaccines are made of weakened virus, which will not cause disease. The vaccine will stimulate the immune system, and the immune system will remember the specific character of the virus. Next time when the real virus invades, the immune system can quickly recognize it and destroy it before it cause serious illness.

However, compared to other organisms, virus mutated very fast. Sometimes there are a lot of variants, and it is a challenge to produce effective vaccines for all the variants. For example, the Influenza virus, next year the virus strain may be different, and you may need to have a new shot of vaccine.

There are thousands of virus genomes have been sequenenced and made public to all researchers. Click here for the detailed virus sequencing project.

» Bacteria and Diseases