Free Viruses MCQs with Answers

21. New influenza vaccines are needed every few years because

• A. immunity produced by the vaccine lasts only days
• B. influenza virus infects only elderly people
• C. antibiotics destroy the vaccine
• D. the surface antigens of the virus mutate frequently

Explanation: Influenza's RNA genome mutates rapidly and its haemagglutinin and neuraminidase surface proteins keep changing, so last year's antibodies may not recognise this year's strain. This antigenic drift is the reason vaccination is reviewed every season.

22. Dengue fever is transmitted to humans by

• A. contaminated drinking water
• B. the bite of Aedes mosquitoes
• C. coughing and sneezing
• D. eating contaminated food

Explanation: Aedes aegypti and Aedes albopictus mosquitoes carry the dengue virus between people, which is why outbreaks follow the mosquito season and stagnant water. Water and food routes spread gut infections such as cholera, not dengue.

23. Antibiotics are useless against viral diseases because viruses

• A. lack their own metabolism and the structures that antibiotics target
• B. are too small for drugs to reach
• C. live only in the soil
• D. are destroyed by stomach acid

Explanation: Antibiotics attack bacterial machinery such as cell walls and 70S ribosomes, and viruses possess neither, borrowing the host's instead. Antivirals must therefore target viral enzymes like reverse transcriptase, which is why far fewer of them exist.

24. Rabies is transmitted to humans mainly through

• A. mosquito bites
• B. contaminated milk
• C. airborne droplets
• D. the bite of an infected animal such as a dog

Explanation: The rabies virus travels in the saliva of infected mammals and enters through a bite, then moves along nerves to the brain, causing hydrophobia. Prompt vaccination after a bite prevents the disease once symptoms appear, when it is almost always fatal.

25. Which hepatitis virus is transmitted mainly through contaminated food and water?

• A. Hepatitis B
• B. Hepatitis C
• C. Hepatitis A
• D. Hepatitis D

Explanation: Hepatitis A, like hepatitis E, spreads by the faecal oral route through polluted food and water and usually resolves on its own. Hepatitis B and C travel in blood and body fluids and can cause chronic liver disease, so the route matters for prevention.

26. Viroids differ from viruses in that viroids

• A. consist only of short circular RNA with no protein coat
• B. contain DNA genomes
• C. are larger than any virus
• D. infect only animals

Explanation: Viroids are the smallest known infectious agents, naked circles of RNA that cause plant diseases such as potato spindle tuber. Viruses at least carry a protein capsid, so viroids are simpler still.

27. The correct sequence of the steps of the lytic cycle is

• A. release, assembly, biosynthesis, penetration, attachment
• B. attachment, penetration, biosynthesis, assembly, release
• C. biosynthesis, attachment, release, penetration, assembly
• D. penetration, release, attachment, assembly, biosynthesis

Explanation: The phage first attaches by its tail fibres, injects its DNA, then commandeers the host to make viral parts, assembles complete particles and finally releases them by lysing the cell. Any sequence that starts later in the list reverses cause and effect.

28. The enzyme HIV carries inside its capsid to make a DNA copy of its RNA genome is

• A. DNA ligase
• B. RNA polymerase
• C. reverse transcriptase
• D. restriction endonuclease

Explanation: Reverse transcriptase copies the viral RNA into DNA, which then integrates into the host chromosome. The direction is the reverse of normal transcription, which is exactly what the name means, and it is this enzyme that drugs like AZT target.

29. HIV attaches to its target cell by binding to the

• A. antibodies on the cell surface
• B. MHC class one molecules
• C. antigen receptors of B lymphocytes
• D. CD4 receptors on helper T lymphocytes

Explanation: The viral glycoprotein gp120 fits the CD4 receptor, which is why helper T cells and macrophages, the CD4 bearing cells, are the targets. Destroying helper T cells collapses both arms of specific immunity, producing AIDS.

30. The interval between HIV infection and the appearance of detectable antibodies in the blood is called the

• A. window period
• B. latent phase of AIDS
• C. incubation of tuberculosis
• D. vaccination period

Explanation: During the window period, which may last weeks to a few months, antibody tests can miss the infection even though the person is already infectious. This is why blood banks also use nucleic acid tests to shorten the risk.