Viruses are smallest infectious agent consisting essentially of nucleic acids like RNA or DNA enclosed in a protein coat or capsid. They are too small to be seen with a standard light microscope. It is an obligate parasite, which means they penetrate the host cells and basically program the cell to assemble new virus particles. They are also sometimes referred to as hijackers; they invade the normal living cell and use those cells to multiply and to produce more viruses like themselves.

The viruses those are pathogenic, may infect the body through various routes such as respiratory tract, ingestion, insect and animal bites and finally may transmit through sexual activities.

The study of the manner in which viruses cause disease is viral pathogenesis. The degree to which a virus causes disease is its virulence.

Types of viruses on the basis of their nucleic acid content:

Viral infection and its symptoms:

Viral infections are caused due to the presence of a virus particle inside the host body. The word virus itself is taken from Latin where it means toxin or poison.

With a viral infection we usually associate high and long lasting fever but some other symptoms can also be noticed like:

• Fatigue
• Headache
• Sneezing
• Depending on the virus:
• Coughing
• Diarrhea
• Abdomen pain
• Sore throat
• Rashes

Transmission of the viral particles:

There are various routes of the transmission of the virus into the body of the host but the most common of them is through droplet contact when the person sneezes or coughs. The virus remains suspended in the droplets and may enter through respiratory tract. There are a variety of other modes also:

• Fecal-oral: due to improper washing of hands leading to spread of disease.
• Vector borne or direct contact: it includes touching of the common objects like telephone booth.
• Sexual contact: One of the most common modes of transmission of viral infection.
• Blood transfusion: By use of shared needles/syringes
• Saliva
• Breast milk
• Bodily fluids

Common diseases caused by virus:

• Common Cold: It’s the most common illness known worldwide. There are about 200 different viruses that have been associated with the cold. The most common viruses are adenovirus and corona virus.
• Influenza virus: also called flu, it is a respiratory infection caused by several different flu viruses. The symptoms of flu and cold usually are same but a few differences like high temperature and extreme body aches associated with flu separates them.
• Chicken pox: It is a highly contagious disease. The major symptoms being rashes, blisters that break open. The disease is often accompanied by high fever, fatigue and loss of appetite.
• HPV:  Human Papilloma Virus, this causes warts of all types. This virus is transmitted through sexual contact in male and female .The percentage is so high that 50% of men and women get it at some point of their life. It is also involved in causing cervical cancer.
• Other diseases like Herpes, AIDS, Meningitis, Measles, Mumps, Polio, and Rabies are all caused due to viruses.

Diagnosis: Generally the history of severe muscle and joint pain before fever and also the presence of skin rashes and lymph gland swelling help the physician to determine the presence of viral infection. The various laboratory tests are done to clarify that whether the symptoms are due to a virus or are due to a bacterial infection. These tests include:

• ELISA
• Hemagglutination  assay
• Cell culture
• Antibody detection
• Nucleic acid detection
• Visual inspection
• Histology( post mortem)

Treatment:

Viruses have a self-limited life, so the treatment is usually done to reduce the symptoms only. Anti pyretic and analgesic drugs are commonly used. Antiviral drugs are a class of medication used specifically for treating viral infections.

The major idea behind modern antiviral drug designing is to identify viral proteins or parts of proteins that can be disabled. These proteins termed as targets should be common around many classes of viruses, so that the single drug may gain a broad spectrum.

Natural antiviral compounds: Certain plants and their products also act as good anti viral compound. Herbal preparations that strengthen the immune system may help the body fight off invading viruses that could otherwise cause infection.

• Glycyrrhizin
• Gossypol
• Hypericin
• Inophyllum B
• Calanotide A&B
• Vidarabine

Certain herbs like cloves, ginger, lemon, licorice, olive leaf, oregano, shiitake and Reishi mushrooms prove to be highly useful in protecting against viruses and boosting up the immune system.

Precautions and preventions:

The following points must be kept in mind in order to keep oneself away from the viral infection and also avoid the spreading of the disease:

• One should try covering mouth when coughing/sneezing with a cloth or with hands.
• Proper washing of hands after touching a publically used object.
• Avoiding close contact with the sick person may help you staying away from the infection.
• Avoiding contaminated food/water.
• Improved sanitation
• Avoiding shared needles/syringes for blood transfusion or drug delivery.
• Practicing safe sex can be really helpful in avoiding STDs.