Microscopic, single-celled organisms that cause disease in people either by invading the tissues or by producing toxins.

• A. Bacteria
• B. Viruses
• C. Fungi
• D. Algae

Answer: (A)

Bacteria fit this description because they are microscopic, single-celled organisms capable of causing disease by either invading tissues or releasing toxins. Their simple cell structure (prokaryotic) lets them colonize tissues directly, leading to infections, or produce exotoxins and endotoxins that disrupt normal bodily functions and cause illness. Viruses are not cells and rely on living host cells to replicate, so they don’t match the idea of a single-celled organism. Fungi can be single-celled (yeasts) or multicellular, and while some cause disease, the core image of a microscopic, single-celled invader producing toxins is more characteristic of bacteria. Algae are generally not human pathogens in the same way. So the description most accurately points to bacteria.