Viral Gastroenteritis: The patient's symptoms of fever and nausea for 7 days, along with a relatively mild decrease in hematocrit and a slight decrease in absolute lymphocytes, could suggest a viral infection. Many viral gastroenteritis cases present with these symptoms and can have mild alterations in blood counts due to the infection and dehydration.

Bacterial Gastroenteritis: Similar to viral gastroenteritis but caused by bacteria. The symptoms can be more severe, and the decrease in lymphocytes might be more pronounced in some cases.
Influenza or Other Respiratory Viruses: Although the primary complaint is gastrointestinal, influenza and other respiratory viruses can cause systemic symptoms including nausea and fever. The mild lymphopenia could be consistent with a viral infection.
Food Poisoning: Caused by the consumption of contaminated food, leading to symptoms like fever and nausea. The blood work might not be significantly altered but could show signs of dehydration or a mild stress response.

Appendicitis: Although the CBC is not highly suggestive, appendicitis can present with non-specific symptoms like fever and nausea. It's crucial to consider this diagnosis due to its potential for severe complications if not promptly treated.
Sepsis: Any infection that leads to a systemic inflammatory response syndrome (SIRS) can cause sepsis. The patient's symptoms and lab results are not strongly indicative, but sepsis is a life-threatening condition that must be considered, especially if the patient shows signs of severity like hypotension, organ dysfunction, or altered mental status.
HIV Serconversion Illness: The patient's lymphopenia could be a clue, as HIV seroconversion can present with a variety of non-specific symptoms including fever, nausea, and changes in lymphocyte counts.

Lymphoma or Leukemia: Although very rare and not directly suggested by the information provided, any significant or unexplained alteration in blood cell counts warrants consideration of hematologic malignancies. However, the mild nature of the lymphopenia and the absence of other cytopenias or blasts make this less likely.
Tuberculosis: Can present with non-specific symptoms like fever and weight loss. While less common, it should be considered, especially if the patient has risk factors for TB exposure or if other diagnoses are ruled out.
Brucellosis: A zoonotic infection that can cause fever, nausea, and changes in blood cell counts. It's rare but should be considered if the patient has a history of exposure to animals or unpasteurized dairy products.