NDA (II) 2016 General Ability Test · Question 71 of 150

Correct — D, they spend a lot of time inside the host's cells. A virus cannot multiply on its own; it must get inside a living cell and hijack that cell's machinery to make copies of itself. For nearly the whole of an infection, therefore, the virus is not floating in the blood where a drug could reach it cleanly, but sheltering within our own cells and using our own enzymes. That is what makes it so hard to kill. Almost anything powerful enough to destroy the virus in that position also destroys the cell hosting it, so an antiviral drug has to find the very few steps that belong to the virus alone and not to us — which is why antivirals are far fewer and far more specific than antibacterial drugs, and why prevention by vaccination has done more against viral disease than treatment ever has.

• (a)consist of tough protein coat — The protein coat, or capsid, is not especially tough. Many viruses carry a fatty envelope over it, and that envelope is easily torn apart — which is exactly why washing with soap is such an effective defence against them.
• (b)are very small in size — Size is no protection. Bacteria are larger than viruses and we kill them routinely, while molecules of a drug are smaller still and go wherever the blood goes. Nothing about being small keeps a virus out of harm's way.
• (c)lack cellular structure — The strongest distractor, because the statement itself is true and it does explain something important — having no cell wall, no ribosomes and no metabolism of its own is precisely why antibiotics have no target in a virus. But that explains why one class of drug fails, not why viruses are hard to destroy in general. A virus particle sitting on a surface is killed by heat, alcohol or bleach within moments; it becomes hard to reach only once it is inside a cell, which is what option (d) says.

A virus is a package of genetic material inside a protein coat, sometimes wrapped in a lipid envelope. It has no cytoplasm, no ribosomes and no metabolism, so outside a host it is inert — closer to a chemical than to an organism. Inside a host cell it comes to life, redirecting the cell's own enzymes and ribosomes to build new virus particles. That total dependence on the host is both the definition of a virus and the reason it is difficult to attack.

The question asks specifically why viruses are difficult to kill, and three of the options describe true properties of viruses without answering that question. Test each one by asking whether it would still be hard to destroy the virus if the property were removed. Small size, a protein coat and the absence of cell structure all leave the virus perfectly vulnerable on a doorknob or a tabletop. Only its residence inside our own cells creates the real problem, which is one of selectivity — hitting the invader without hitting the host. Option (c) deserves respect rather than dismissal, because it correctly explains why antibiotics fail; NDA itself set a question on that point in 2020, which is cited below.

• A virus can multiply only inside a living host cell, using the host's own enzymes and ribosomes.
• Because the virus sits inside our cells, a drug that destroys it there risks destroying the cell too, which is why selective antiviral drugs are hard to design.
• Outside a host, virus particles are comparatively fragile and are inactivated by soap, heat, alcohol and ultraviolet light.
• Antibiotics act on bacterial cell walls, ribosomes and metabolic pathways, none of which a virus possesses.
• Vaccination, which prepares the immune system before infection, remains the most effective control for viral disease.

• Picking the option that states a true fact about viruses rather than the option that answers the question asked.
• Believing that viruses are physically indestructible, when they are readily inactivated outside a host.

NDA GAT returns to viruses regularly, often through a not-true statement or a why question, so learn the virus by what it lacks as much as by what it has.

• practice — not a real PYQ

  Antibiotics are ineffective against viral infections chiefly because viruses

  • (a)are too small for antibiotics to reach
  • (b)lack the cell structures and metabolic pathways that antibiotics act upon
  • (c)reproduce too quickly for any drug to keep pace
  • (d)are protected by a coat that antibiotics cannot penetrate

  Answer(b) lack the cell structures and metabolic pathways that antibiotics act upon — there is simply nothing for the drug to attack.

• practice — not a real PYQ

  Outside a living host, a virus is best described as

  • (a)an actively growing organism
  • (b)essentially inert, behaving like a chemical substance
  • (c)a single-celled organism with its own metabolism
  • (d)a spore-forming bacterium

  Answer(b) essentially inert, behaving like a chemical substance — it shows no life activity until it enters a cell.