Viral Infection Review from
Biology Questions and Answers
Learn the Main Viral Infections Here
1. What are some human diseases caused by virus and what are their respective modes of transmission?
The main viral diseases transmitted by respiratory secretions (sneezes, cough) and by saliva drops are flu, mumps, smallpox (variola, already considered eradicated), rubella, measles, SARS. Main viral diseases transmitted through blood or sexual contact are AIDS, hepatitis B, hepatitis C, HPV, ebola hemorrhagic fever. Main viral diseases transmitted by animal vectors are rabies, dengue fever, yellow fever. Some viral diseases transmitted by fecal-oral route, including contaminated food, are hepatitis A, poliomyelitis (disease almost eradicated in many parts of the world).
2. What is the virus that causes flu? Why doesn't the body produce permanent immunity against that virus? How does the vaccine against flu work?
Flu is a disease caused by the influenza virus, a highly mutant DNA virus. Due to the high mutation rate of the virus, that forms many different strains, flu always presents epidemic features in affected populations and people may have several flu episodes during life (the immune response made from previous infections is not efficient in future infections).
The vaccine against flu is a vaccine made of attenuated virus of three different strains. Each year the WHO (World Health Organization) researches and determines which are the strains that should compose the vaccine. This is a strategy to face the high mutation rate of the virus.
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3. Why is rubella during gestation a threat to the fetus?
If occurring during gestation rubella is a dangerous disease because the virus crosses the placenta and contaminates the fetus. The fetus then develops congenital rubella, a teratogenic (cause of malformations) disease.
Congenital rubella may be prevented by vaccination. Doctor must always be consulted before vaccination.
4. What are the main available vaccines against poliomyelitis?
The vaccines used against poliomyelitis are the Sabin vaccine and the Salk vaccine. The Sabin vaccine contains attenuated virus and is taken through oral drops. The Salk vaccine is made of dead virus and is administered by injection.
5. Is rabies caused by virus or by bacteria? How is it transmitted? Which organs and tissues are affected by the disease? How is it prevented and treated?
Rabies, also known as hydrophobia, is a viral disease. It is found in dogs, cats, bats and other wild mammals. The transmission to humans occurs through the saliva of contaminated animals, mainly through bites.
The rabies virus is neurotropic and attacks the central nervous system in a fast and lethal fashion. The prevention of the disease is done by prophylactic vaccination of animals and humans. The treatment is done by anti-rabies serum containing specific antibodies against the virus.
6. What is the difference between smallpox (variola) and measles?
Smallpox is a viral infection like measles. Smallpox is transmitted by respiratory secretions, saliva and objects in contact with contaminated patients. The disease is characterized by the appearance of numerous vesicles on the skin of the face, trunk and limbs, that can suppurate and form crusts; smallpox complications can lead to death. Measles is transmitted by saliva drops and respiratory secretions too. It is characterized by exanthems (red spots on the skin), fever, malaise and lymphadenomegaly (enlarged lymphnodes). If not treated measles complications can cause death.
Smallpox killed thousands of people around the world in the end of the 19th century and the beginning of the 20th century but today it is almost eradicated by vaccination. Nowadays however a great fear is the possibility of use of smallpox virus in biological weapons and by bioterrorists. Measles is not eradicated and affects millions of people each year. Both diseases can be prevented by vaccination.
7. What are the three main types of viral hepatitis?
There are many types of viral hepatitis. The most important epidemiologically are hepatitis A, hepatitis B and hepatitis C.
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8. What are the modes of transmission, main signs and symptoms and treatments of hepatitis A?
Hepatitis A is an acute disease of low mortality caused by the hepatitis A virus (an RNA virus). It is transmitted by fecal-oral route often through contamination of foods like vegetables and sea-food. The virus attacks the liver and its incubation period varies between 15 and 45 days. After incubation the symptoms are fever, headache, abdominal pain, malaise, nausea and vomiting and the main signs are jaundice, hepatomegaly (enlargement of the liver) and darkened urine (due to excessive bile in blood). Blood tests show increased levels of hepatic enzymes caused by injuries to liver cells. Hepatitis A often heals naturally after 4 to 6 weeks.
9. What are the modes of transmission, main signs and symptoms and treatments of hepatitis B?
Hepatitis B is a disease caused by a DNA virus. The transmission is by blood (wounds, sexual relations, transfusions, accidents with contaminated material, etc.). The most common form of the disease is chronic, the infected patient carries the virus for life and the liver gradually suffers injuries that can lead to cirrhosis or even to hepatic cancer. Another form of the disease is acute, sometimes fulminating, with symptoms similar to those of hepatitis A but more severe and sometimes lethal. There are persons that are asymptomatic carriers of the virus. The level of hepatic injury is assessed by blood tests of hepatic enzymes and by hepatic biopsy (extraction of small samples of tissue to be examined under the microscope). Viral replication can be estimated through PCR (polymerase chain reaction).
10. What are the modes of transmission, main signs and symptoms and treatments of hepatitis C?
Hepatitis C is caused by an RNA virus and transmitted through blood (like hepatitis B, through wounds, sexual relations, transfusions, accidents with contaminated material, etc.). The usual manifestation is chronic and many people that have the virus are asymptomatic. The liver gradually undergoes injuries and some patients evolve to cirrhosis, hepatic cancer and death. Today interferon use is an attempt to control replication of the hepatitis C virus. The assessment of the disease is made periodically by hepatic biopsy and by blood tests of hepatic enzymes. Viral replication can be estimated through PCR (polymerase chain reaction).
11. Are there non viral hepatitides?
Hepatitis is a generic name for inflammation of the liver. There are bacterial hepatitides, for example, in lepstopirosis, and toxic hepatitides, for example by alcohol, medicines or inhaled chemical products.
12. What are the main human viral diseases transmitted by mosquitoes?
The main human viral infections transmitted by mosquitoes are dengue fever and yellow fever.
13. What is dengue?
Dengue, or dengue fever, is an epidemic disease in some countries (for example, in Brazil), and its most dangerous form is hemorrhagic dengue. It is caused by four different but related viruses, the reason that it is difficult for an already infected person to become immunized against the disease. Dengue is prevalent in tropical and subtropical regions of Asia, Africa and South America and it is transmitted by the bite of the Aedes aegypti mosquito, a diurnal mosquito. The viral infection causes varied symptoms similar to flu, like fever, malaise, myalgia (muscle pain); in hemorrhagic dengue high fever, with or without convulsions, and hemorrhages may occur leading to circulatory failure and death. There is no vaccine against dengue since there are four agent viruses and it is supposed that the severity of the disease is greater when the patient is already protected against one type of virus and is infected by another. The best prophylactic method is to eliminate the vector mosquito.
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14. What is yellow fever?
Yellow fever is a viral infection that occurs mainly in Central Africa and in the Amazon region of South America. It is prevented through vaccination and transmitted by many species of mosquitoes of the Aedes genus, including Aedes aegypti e Haemogogus. The infection causes clinical manifestations that range from asymptomatic cases to lethal fulminating cases. Generally the disease begins with fever, chills, malaise, headache, nausea and evolves to jaundice (increase of bilirubin in blood, after which the disease is named), mucosal and internal hemorrhages, hemorrhagic vomiting and renal failure.
Prevention is done by regular mass vaccination and vaccination of travelers to endemic areas. The combat against the vector mosquito is also an important prophylactic measure.
15. Why is it difficult to produce efficient vaccines against a viral infection like dengue and AIDS?
It is difficult to make vaccines against dengue because there are four different types of viruses that cause the disease (DEN1, DEN2, DEN3 and DEN4) and it is supposed that the protection against one of them aggravates the clinical manifestation when the person is later infected by the another dengue virus.
In the case of HIV, the production of a vaccine is difficult because the virus is highly mutant and evades the antibody action.
16. Is a viral infection treated with the same kind of drug that treats bacterial infections?
Antibacterial drugs, potent against a great variety of bacteria, are not effective against viruses, which are intracellular parasites. A viral infection is difficult to treat since anti-viral drugs are too specific and have limited efficiency. In general the anti-viral drugs reduce the viral load (number of virus) relieving symptoms.
The anti-virals (and anti-retrovirals, drugs that act against RNA virals) often inhibit the action of specific enzymes that participate in the virus life cycle.
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