NEET Biology Human Health And Disease / मानव स्वास्थ्य और रोग Diseases caused by Viruses

Diseases caused by Viruses

(a) Important Diseases caused by Viruses: The human diseases caused by viruses include influenza, chickenpox, smallpox measles, rabies, mumps, polio, trachoma, hepatitis and AIDS.

      (1) Influenza: Influenza, commonly called flu, is a highly infectious disease, which has still not been conquered. It is caused by many kinds of viruses, such as myxovirus. The latter affect the mucous membrane of nose, throat and upper respiratory tract. The common symptoms are discharge from the nose, sneezing, fever, body aches, coughing and general weakness. The infection spreads by discharges from the nose and throat. The incubation period is just from 24-72 hours. Influenza generally lasts for 4 or 5 days. Rest quickens the recovery. If neglected, complications like pneumonia, bronchitis and ear infection may develop. There is no vaccine for influenza.

        Influenza tends to occur in epidemic or pandemic form with varying virulence.

      (2) Chickenpox : It is a common, relatively mild, highly contagious disease of children, generally under 10 years of age. It is caused by a virus called chickenpox virus (varicella zoster). Fever, aches and general discomfort are the symptoms. Dewdrop-like sores appear in successive crops, first on the trunk. The sores open and a fluid seeps out a short time later. The disease spreads by direct contact with skin sores or with clothes and other articles soiled with discharges from sores. Incubation period is 2-5 weeks. The sores heal without leaving scars. Preventive measure is isolation of the patient till all crusts fall off. One attack of chickenpox ordinarily gives permanent immunity to the disease. There is no vaccine against chickenpox. Chickenpox is rarely fatal, but in adults attack could be severe.

      (3) Smallpox : Smallpox is an acute, highly communicable disease. It is caused by a virus named variola virus. It starts as a sudden onset of high fever accompanied by headache, backache, and pains all over the body. Rash appears on the 3rd or 4th day of illness. The rash gradually changes into pustules (pimples) containing clear fluid. The pustules finally form scabs which fall off by the 3rd week. The scabs leave behind permanent pitted scars, the pockmarks, on the skin. The disease may lead to blindness.

        Smallpox spreads by exudate from pustules on the skin of the infected persons. It also spreads by oral and nasal discharges during coughing and sneezing, and by contact with the clothes of the patient soiled with discharges. Its incubation period is about 12 days. It is very serious, disfiguring and highly fatal disease. It has now been largely controlled through vaccination. Smallpox vaccine was first prepared by Edward Jenner in 1798.

      (4) Measles : Measles is one of the most prevalent and serious diseases of children, generally 3-5years old. It is caused  by a virus named rubeola virus. It is characterized by fever, inflammation of nasal mucous membrane, red watery eyes sensitive to light, flushed face, loss of appetite, followed by a typical rash, i.e., eruption of small red spots (rubeola). Infection is spread by discharges from nose and throat (droplet infection). The incubation period is about 10 days. One attack of measles gives life-long immunity. Vaccine which produces active immunity is available.

        Patients of measles are likely to catch secondary infection of pneumonia.

      (5) Rabies (Hydrophobia) : Rabies is a 100% fatal disease. It is caused by a rabies virus. The virus enters the human body with saliva of an infected (rabid) animal, generally by the bite of a dog but also of cat. Virus induces biting behaviour in its victim. Fear of water is the main symptom, hence hydrophobia. Incubation period is commonly 1-3 months, but may vary from 10 days to one year. This long period of incubation makes it possible for a rabies vaccination after a bite to develop immunity and prevent the appearance of the disease. The virus of rabies destroys the brain and spinal cord cells. The patient feels severe headache, high fever, restlessness and inability to swallow even fluids due to choked throat. The main preventive measures are eradication of stray dogs and cats and compulsory immunization of pet dogs and cats. The pet should be watched for 10 days after it has bitten someone to make sure that it does not have rabies. Symptoms of rabies in dogs are madness, changed voice and excessive salivation. Rabid dogs should be immediately killed. Treatment of rabies was discovered by Louis Pasteur. It involves a series of 14 injections given after the bite of a dog.

      (6) Mumps (Infectious Perotitis) : Mumps is an acute communicable disease, generally of children. It is caused by a paramyxo virus, which has preference for salivary glands but may attack other glands of the body also. It is characterized by painful enlargement of one or both the parotid glands. The latter lie below the pinnae. The patient has high fever and difficulty in opening mouth. The virus is spread by discharges from the throat of an infected person (droplet infection) and by direct contact. The incubation period varies from 12-26 days. In adults testes and ovaries may also become inflamed. Infection of testes may cause sterility. One attack of mumps gives life-long immunity.

      (7) Poliomyelitis or Polio (Infantile Paralysis) : Polio is most prevalent in hot, dry weather. Its common name is inappropriate as it is not necessarily a disease of infants nor does it always cause paralysis. It is caused by a virus known as polio virus. This virus causes inflammation of nervous system and stiffness of the neck. It also destroys motor nerve cells in the spinal cord. Muscles fail to work and shrink due to lack of nerve impulses. This may cause paralysis of limbs in some cases. The virus enters the digestive tract with contaminated food and water and multiplies in the intestinal cells. It then passes into blood stream and lymphatic system, and finally reaches the spinal cord where it starts multiplication. Incubation period is 7-14 days. A patient who recovers from polio has a life time immunity. Now oral vaccine of polio is available.

        The polio virus may attack the respiratory centres in the brain. This may stop nerve impulses to the diaphragm and breathing may stop. Then artificial breathing with ?iron lung? becomes necessary.

        As polio cripples the children for life and is not curable, its prevention by oral vaccine is essential.

        Oral vaccines are developed by Jonas Salk and Albert Sabin in 1940. Public pulse polio immunization programme is organized in India for eradicating polio in 1996.

      (8) Trachoma : Trachoma is a chronic inflammatory disease of the eyes all over the world. It is caused by a pathogen formerly considered a virus, new regarded an agent occupying a position intermediate between rickettsiae and viruses and named Chlamydia trachomatis. The latter affects eyelids, conjunctiva and cornea. It causes granules and may lead to blindness. The common symptoms are inflammation, discomfort and discharge from the eyes. Infection spreads by direct contact, by use of towels, pillows and handkerchiefs of the patients and by flies. The incubation period is 5-12 days. Trachoma can be controlled with antibiotics in early stages. Severe infection needs operation-involving scrapping of granules. Trachoma accounts for 5 percent of the blind cases in India.

      (9) Dengue Fever (Backbone fever):

        Dengue fever is a tropical viral disease spread by the tiger mosquito Aedes aegypti.

      Dengue fever/Dengue Haemorrhagic Fever (DF/DHF), one of the dangerous diseases, can be detected by tourniquet test.

        The symptoms of this disease include high fever, severe frontal headache, pain behind eyes, muscles and joint pain, loss of appetite, Measles-like rashes over chest and upper limbs, nausea and vomiting.

        Dengue can be prevented through elimination of mosquito breeding places.

      (10) Yellow fever

               Yellow fever, caused by an arbovirus is a haemorrhagic disease transmitted by the infected Aedes aegypti.

               Symptoms of yellow fever are headache, fever, vomiting, rapture of veins in kidneys, spleen, liver etc.

               In severe cases, the skin of sufferer becomes yellow from jaundice? hence the name yellow fever.

               Max Theiler in 1951 got Nobel Prize for the development of vaccine for yellow fever.

Viral Diseases in Humans