Legionnaires’ disease usually starts like a bad flu or pneumonia and then gets more severe over a couple of days.

Early symptoms (first signs)

These are the common early clues doctors look for.

  • High fever (often 38.5–40°C / 101–104°F), feeling very feverish or having chills.
  • Headache that can be moderate to severe.
  • Muscle aches and general body pain.
  • Feeling very tired or weak (fatigue).

Symptoms usually start 2–14 days after breathing in water droplets containing Legionella bacteria.

Lung (respiratory) symptoms

As the infection settles in the lungs, it behaves like a serious pneumonia.

  • Persistent cough, which can be dry or bring up mucus or, less commonly, blood.
  • Shortness of breath or trouble breathing, especially with activity.
  • Chest pain, sometimes sharp or worse when you take a deep breath or cough.

If someone has a high fever plus a new cough and feels very unwell, doctors often consider Legionnaires’ as one possible cause, especially if there was exposure to things like hot tubs, cooling towers, fountains, or large plumbing systems.

Digestive and brain-related symptoms

One important feature of Legionnaires’ disease is that it often affects the gut and the brain as well as the lungs.

  • Diarrhea.
  • Nausea and sometimes vomiting.
  • Loss of appetite and feeling generally unwell.
  • Confusion, difficulty thinking clearly, or changes in mental state.
  • Weakness and sometimes poor coordination.

These “extra” symptoms (especially diarrhea plus confusion) are part of the reason doctors may suspect Legionnaires’ rather than a more typical pneumonia.

How fast it can get serious

  • Symptoms often start mildly but can worsen over 2–3 days into severe pneumonia with major breathing difficulty.
  • Without prompt antibiotics, Legionnaires’ disease can be life‑threatening, especially in older adults, smokers, or people with weak immune systems or chronic lung disease.

If someone has high fever, cough, shortness of breath, and feels very sick (especially with diarrhea or confusion), they should seek urgent medical care and tell the doctor about any recent exposure to hotel stays, spas, hot tubs, cooling towers, or big building water systems.

TL;DR:
Legionnaires’ disease causes high fever, headache, muscle aches, and then a worsening cough, shortness of breath, and chest pain, often with diarrhea and confusion, usually starting 2–14 days after exposure.

Information gathered from public forums or data available on the internet and portrayed here.