Final answer:
B. bacteria
Antibiotics typically target bacteria by affecting parts of the bacterial cell that are different from human cells. Developing targeted drugs for fungi, protozoa, helminths, and viruses is more challenging due to the close similarity between these organisms' cells and human cells, or because they replicate within human cells.
Step-by-step explanation:
The term "antibiotic" is usually reserved for drugs that target bacteria. Antibiotics are designed to fight bacterial infections, selectively killing or inhibiting the growth of these prokaryotic organisms without harming the host's cells. This is possible because bacterial cells have unique parts that can be specifically targeted without affecting human cells. Some parts that antibiotics may target include the bacterial cell wall, protein synthesis machinery, and DNA replication processes.
The development of drugs targeting different classes of pathogens varies in complexity. Antimicrobial drugs that target fungi, protozoa, and helminths face challenges due to the cells of these organisms being similar to human cells, making selective toxicity more difficult. On the other hand, it is also hard to develop drugs targeting viruses, as they replicate within human host cells.