When it hits
Day 50 on the 
Wage Slave's run
No warning comes first.
Starts with Feeling Unwell, ends with 
Epidemic Subsides.
The same on every run: Student, Warehouse Manager
What comes
No zombies: this crisis is about the weather, sickness or pests, not a siege.
From the game files1
Crisis 101
| id | 101 |
|---|---|
| CrisisID | 101 |
| StartEvent | [40600] |
| EndEvent | [40601] |
| Des | Crisis_Des_101 |
| Des_Local | 疫病 |