All right, let's discuss this important question many people are concerned about.

When does the "acute phase" of HIV infection typically occur? What are the flu-like symptoms?

Think of this so-called "acute phase" as the stage where your body's immune system first detects the HIV virus as an "intruder," immediately sounds the alarm, and mobilizes all its forces to fight this "initial encounter."

When does the "acute phase" occur?

Timing: It typically occurs 2 to 4 weeks after exposure. Of course, this isn't absolute—some people might experience it slightly earlier or later—but the vast majority fall within this timeframe.
Why this timing? After the virus enters the body, it needs time to replicate and spread. When the viral load reaches a certain level, the immune system finally "notices" it and launches a strong counterattack. The external manifestations of this "battle" are the symptoms discussed below.
Crucial Reminder: Not everyone experiences acute phase symptoms! About 50%-90% of infected individuals develop symptoms. Conversely, that means some people (possibly nearly half) may have a "silent" acute phase with no noticeable symptoms. Therefore, the absence of symptoms does not mean there is no infection.

What are the flu-like symptoms?

Symptoms during the acute phase are varied and can strongly resemble those of a severe cold, flu, or even other viral infections (like EBV), making them easily confused. The most common ones include:

Fever: One of the most common symptoms. It might be low-grade or high, with temperatures above 38.5°C, and typically lasts for several days.
Severe Fatigue, Lack of Energy: Feeling completely drained and listless all day, lacking energy for anything. This level of fatigue is much more profound than normal tiredness.
Sore Throat: Painful, scratchy throat, similar to a common cold or tonsillitis.
Muscle and Joint Pain: Aching muscles and joints, feeling similar to having the flu.
Swollen Lymph Nodes: A relatively characteristic symptom. You might notice swollen lumps under the skin in your neck, underarms, or groin, which may be slightly tender to the touch.
Rash: Another relatively characteristic sign. Usually appears as red or dark spots or patches on the chest, back, and face, and is not particularly itchy.
Headache, Diarrhea, Nausea/Vomiting: Some people may also experience gastrointestinal or neurological symptoms like these.

The Most Important Point: Do NOT Self-Diagnose!

Reading this, you might be tempted to compare the list of symptoms to your own situation and become anxious. Please remember:

Symptoms alone cannot tell you if you have HIV.

The reasons are simple:

Symptoms are not specific: All the symptoms listed above could be caused by a cold, the flu, exhaustion, allergies, other viral infections—a variety of conditions.
Not everyone has them: As mentioned, some infected individuals have no symptoms at all.

Therefore, if you've had high-risk exposure and are concerned, the only scientific, definitive, and accurate thing to do is:

Get an HIV test after the "window period."

Don't panic unnecessarily, but also don't become complacent just because you lack symptoms. Testing is the only way to get a clear answer.