Oncogenic viruses history: which virus was the first tumor-inducing (oncogenic) virus to be detected?

Introduction / Context:
Oncogenic viruses are capable of initiating or promoting tumor formation in animals or humans. The historical sequence of their discovery is foundational knowledge in virology and cancer biology. This question asks which virus was first recognized as oncogenic.

Given Data / Assumptions:

• Peyton Rous reported a transmissible agent causing sarcomas in chickens in the early 20th century.
• Later discoveries included Epstein–Barr virus (EBV) and human tumor viruses such as HTLV.
• The question is about the first detected oncogenic virus, not the first human oncogenic virus.

Concept / Approach:
Rous demonstrated that a cell-free filtrate from chicken tumors could induce sarcomas when injected into healthy birds. The agent, later known as Rous sarcoma virus (RSV), is an avian retrovirus and is historically credited as the first tumor virus discovered. EBV (a gammaherpesvirus) was linked later to Burkitt lymphoma and nasopharyngeal carcinoma; HTLV and HSV-2 associations came even later or are more limited/controversial in humans.

Step-by-Step Solution:

Verification / Alternative check:
Historical reviews of tumor virology consistently cite RSV as the first tumor-causing virus discovered.

Why Other Options Are Wrong:

• Epstein–Barr virus: Discovered later; not first.
• HSV-2: Associations are not the earliest and remain complex.
• HTLV: Identified later as a human oncovirus; still not the first.

Common Pitfalls:
Confusing “first detected” with “first human tumor virus”; assuming human relevance is implied by “oncogenic.”

Final Answer:
Rous sarcoma virus.