Viruses like adenovirus, mono, gastroenteritis and bacterial infections like pneumonia, strep, tick borne illnesses like Lyme disease and even early hepatitis and HIV can mimic flu like symptoms.
Even COVID can mimic flu like symptoms.
COVID-19 shares symptoms of the flu like fever, cough, body aches and sore throat.
RSV (Respiratory Syncytial Virus) causes cold-like symptoms, potentially severe.
Mononucleosis (Mono) causes extreme fatigue, fever, sore throat, swollen glands.
Common Cold/Adenovirus causes mild, but can sometimes feel more intense.
Viral Gastroenteritis causes stomach flu with fever, aches, vomiting/diarrhea.
HIV (Acute) causes flu-like illness can be the first sign of infection.
Strep Throat causes sore throat, fever, fatigue, headache but less cough.
Pneumonia causes cough, fever, chills, shortness of breath and Lyme disease causes fever, headache, fatigue and muscle aches after a tick bite.
There's also a new strain of flu going around right now which is the H3N2 flu strain and it's making some people feel worse than the other flu viruses.
The new strain of the flu virus is currently more aggressive than the older flu viruses.
The new flu strain is showing typical influenza symptoms which include sudden fever, cough, sore throat, runny/stuffy nose, body aches, headache, and fatigue, with some experiencing vomiting/diarrhea (more common in kids).
Key indicators of the flu including the new flu strain are abrupt onset and intense fatigue/aches, but symptoms overlap with COVID-19, so testing helps differentiate, with emergency signs including trouble breathing or confusion.
COVID-19 and the flu also feel very similar, and share many of the same symptoms like fever, cough, fatigue, sore throat, and body aches, making them hard to tell apart just by how you feel, though COVID-19 is more known for taste/smell loss and sometimes diarrhea, while flu often hits suddenly with deep exhaustion and joint pain.
Respiratory adenovirus infections typically also cause mild cold or flu-like symptoms, although they can also cause acute bronchitis, pneumonia, conjunctivitis (pink eye), and acute gastroenteritis (inflammation of the stomach or intestines causing diarrhea, vomiting, nausea, and stomach pain).