What are the characteristics of viruses or parasites?
5.4 Parasitic Nature of Viruses.
live outside living Viruses are obligate parasites i.e., they cannot reproduce mitochondria etc for cells. It is because of making protein and energy. Therefore, viruses need a host cell for their life cycle. Viruses are highly specific concerning their hosts, e.g., HIV attacks on the T4 cells of humans.
The Polio virus infects spinal nerve cells. The hepatitis virus attacks liver cells. Bacteriophages attack only bacteria etc. However, some viruses have a broad range of specificity e.g., rabies virus can infect all mammalian cells.
When any foreign agent enters inside the body it is destroyed or killed by macrophages and neutrophils or antibodies produced by Bursa lymphocytes. But in some capsules, protein and fibrin do not bind by geg (swing) like substances secreted by Bursa lymphocytes which are used by macrophages and neutrophils.
That is why viruses are saved from being phagocytized. Some viruses are covered with host proteins, therefore, the body's immune system is unable to detect them as a foreign body and they remain protected.
Many viruses continuously change their shape and appearance as a result body's immune system and vaccines become ineffective against new types, e.g., influenza and HIV viruses also remain safe in the body when the immune system gets weak as in AIDS.
How do viruses tolerate unfavorable conditions outside the host cell?
Outside the host cell viruses are changed into crystals. In crystal form, they are seen dead and show no activities. Upon reaching the host cell, i.e., in favorable conditions, they become active again and start reproducing by using host enzymes and proteins. The crystals of viruses may be present in saliva, respiratory droplets, feces, etc.
5.5 Viral Diseases
A disease caused by a virus is known as a viral disease. Viruses cause several diseases in plants, animals,s and human beings. A brief introduction to some viral diseases is given below.
Hepatitis:
Hepatitis is the inflammation of the liver (Gk. Hepa Liver, itis = inflammation)
There are different causes of hepatitis such as alcohol, drugs, and toxins. However, hepatitis is mostly caused by viral infections. There are several types of viral hepatitis like A, B, C, D, and E.
Hepatitis A:
It is caused by an RNA virus called HAV. The HAV is a non-enveloped icosahedral-shaped virus that causes a mild, short-term disease. It is transmitted by contact with feces from an infected person and drinking sewage-contaminated water. Vaccine is available for the prevention of HAV but no antiviral therapy is available.
Hepatitis B:
Serum Hepatitis: It is caused by DNA enveloped virus called HBV. It is transmitted by blood, sexual contact, contaminated blood transfusion,n and by infected mothers to their babies, saliva, etc. It may cause liver cirrhosis and death if not treated timely. The vaccine for HBV is available. Alpha interferon and some nucleoside analogs are effective treatments for HBV.
is a chronic and fatal disease, that may cause cirrhosis, hepatocellular carcinoma, and death if left untreated. The mode of transmission is via blood, sexual contact, breastfeeding, sharing needles, toothbrushes, etc. No vaccine is available for HCV, however, antiviral therapy is available usually a combination of interferon and ribavirin is given to the patients.
Hepatitis D:
It is caused by HDV also called delta virus. This virus is only active in the presence of HBV, so it can be treated or prevented by treating HBV. Its mode of transmission is also the same as HBV.cal enveloped viroid.
Hepatitis E:
It is caused by HEV. non-enveloped single-stranded RNA virus. The symptoms of HEV are similar to HAV. But it can be more fulminant in some cases such as pregnancy. No vaccine or antiviral drug is available. Herpes: Two types of herpes viruses cause herpes, i.e., herpes simplex virus I and II.
These are double-stranded DNA viruses having large genomes covered with protein coats and envelopes. Herpes simplex- is known as cold sore while herpes simplex-II is known as genital herpes. Herpes-I is transmitted by saliva while herpes-II is transmitted by sexual contact. The symptoms include water blisters in the skin or mucous membranes of the mouth, lips, nose, genitals, and skin lesions.
Herpes can be treated by using antiviral drugs and may be prevented by avoiding sexual contact and physical contact with infected persons. Poliomyelitis (infantile paralysis) is a highly infectious viral disease that can lead to paralysis, breathing problems, or even death.
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