Virology fundamentals – Unit of measurement: In routine electron microscopy and microbiology, the size of viruses is usually expressed in which unit?

Introduction:
Knowing the typical scale of viruses is a foundational concept in microbiology. Virus particles are smaller than most bacteria and far below the resolving power of light microscopy, which is why electron microscopy and nanometer-scale measurements are commonly used. This question tests whether you can recall the correct unit used for reporting viral dimensions in standard references and laboratory reports.

Given Data / Assumptions:

• Most animal and plant viruses have diameters roughly between 20 nm and 300 nm.
• Light microscopes resolve down to about 200 nm under ideal conditions.
• Electron microscopy routinely measures structures in nanometers.

Concept / Approach:
The metric system offers several units: millimeter (10^-3 m), micrometer (10^-6 m), and nanometer (10^-9 m). Viral dimensions are so small that nanometers are the most practical unit for direct description, comparison, and imaging analysis.

Step-by-Step Solution:
Step 1: Recall that bacteria are typically 0.5–5 µm; viruses are far smaller.Step 2: Many standard examples: parvoviruses ~20–25 nm; adenoviruses ~90–100 nm; poxviruses ~200–300 nm.Step 3: Because 1 µm = 1000 nm, reporting a 90 nm virion as 0.09 µm is less conventional in virology literature.Step 4: Therefore, the best unit is nanometers (nm).

Verification / Alternative check:
Textbooks, CDC/WHO fact sheets, and electron microscopy atlases consistently list virion diameters in nanometers, confirming discipline-wide convention.

Why Other Options Are Wrong:

• Centimeters: far too large for microscopic entities.
• Micrometers: more common for cells and bacteria, not for precise viral sizing.
• Millimeters: inappropriate scale for microscopic particles.
• Angstroms as the primary unit: sometimes used for atomic-scale detail (1 Å = 0.1 nm) but not the standard unit for whole virions.

Common Pitfalls:
Conflating bacterial and viral size ranges; assuming micrometers suffice for all microbes; mixing nm and Å without context.

Final Answer:
Nanometers (nm).