Onset (noun)

1. The beginning of something, especially something unpleasant.
2. The initial stage of a disease or illness.

Origin:

From onset, 'attack, assault', from old english onstæt, from on + -stæt, related to stætan 'to stand'.

Examples:

1. The onset of winter brought with it cold temperatures and snow.
2. The onset of the pandemic led to widespread panic and fear.
3. The onset of the disease was sudden and unexpected.
4. The onset of symptoms can vary from person to person.
5. The onset of puberty is a significant milestone in a young person's life.