**Core Concept**
Lymphomas are a group of cancers that arise from the immune system's lymphatic cells, including B cells, T cells, and natural killer cells. They are characterized by the uncontrolled growth of malignant lymphocytes, leading to various clinical manifestations.

**Why the Correct Answer is Right**
The correct answer will be the statement that does not accurately describe a characteristic of lymphoma. Lymphomas can be broadly classified into two main categories: Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL). HL is characterized by the presence of Reed-Sternberg cells, whereas NHL is a heterogeneous group of lymphoid malignancies. Lymphomas can also be further classified based on their cell of origin, such as B-cell or T-cell lymphomas.

**Why Each Wrong Option is Incorrect**
**Option A:** Lymphomas are always aggressive and have a poor prognosis. This statement is not true because while some lymphomas can be aggressive, others can be indolent and have a more favorable prognosis with treatment.
**Option B:** Lymphomas are only treated with chemotherapy. This statement is not true because lymphomas can be treated with a variety of modalities, including chemotherapy, radiation therapy, immunotherapy, and targeted therapy.
**Option C:** Lymphomas can be caused by viral infections such as HIV and EBV. This statement is not true because while viral infections can increase the risk of developing lymphoma, they are not the primary cause of the disease.
**Option D:** Lymphomas can be diagnosed with a simple blood test. This statement is not true because lymphoma diagnosis typically requires a combination of clinical evaluation, imaging studies, and tissue biopsy.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that lymphomas can present with a wide range of symptoms, including fever, weight loss, and fatigue, and that a tissue biopsy is often necessary for definitive diagnosis.

**Correct Answer: C. Lymphomas can be caused by viral infections such as HIV and EBV.**