**Core Concept**
Bacteriophages, also known as phages, are viruses that infect bacteria. They have been researched for their potential use as therapeutic agents, particularly for treating antibiotic-resistant bacterial infections.

**Why the Correct Answer is Right**
Bacteriophages are most needed for treating bacterial infections that are resistant to conventional antibiotics. This is because phages specifically target bacterial cells, reducing the risk of collateral damage to human cells. The mechanism of action involves phages binding to specific receptors on the bacterial cell surface, followed by injection of phage DNA into the bacterial cell, which ultimately leads to bacterial lysis and death.

**Why Each Wrong Option is Incorrect**
* **Option A:** While bacteriophages have been used in vaccine development, this is not their primary application.
* **Option B:** Gene therapy is a different field that involves introducing healthy copies of a gene into human cells, which is not the primary use of bacteriophages.
* **Option C:** Bacteriophages are not typically used for treating viral infections, as they are specific to bacterial cells.

**Clinical Pearl / High-Yield Fact**
Bacteriophages have been used therapeutically since the 1920s, but their use declined with the advent of antibiotics. However, with the resurgence of antibiotic resistance, phages are being re-examined as a potential treatment option.

**Correct Answer: D. Treating antibiotic-resistant bacterial infections.