What is high altitude sickness? Write its symptoms.

Explanation

### Step-by-Step Solution:

**Step 1: Define High Altitude Sickness**
High altitude sickness, also known as mountain sickness, is a condition that occurs when individuals ascend to high altitudes where the atmospheric pressure is low. This low pressure results in a decrease in the availability of oxygen, which can lead to various illness symptoms.

**Step 2: Explain the Cause**
The primary cause of high altitude sickness is the low atmospheric pressure at elevated altitudes. As one climbs higher, the amount of oxygen in the air decreases, making it difficult for the body to obtain sufficient oxygen. This lack of oxygen can trigger a series of physiological responses that manifest as symptoms of sickness.

...