When it hits
Day 50 on the Student's run
No warning comes first.
Starts with Feeling Unwell, ends with 
Epidemic Subsides.
The same on every run: Wage Slave, Warehouse Manager
What comes
No zombies: this crisis is about the weather, sickness or pests, not a siege.
From the game files1
Crisis 121
| id | 121 |
|---|---|
| CrisisID | 121 |
| StartEvent | [52600] |
| EndEvent | [52601] |
| Des | Crisis_Des_121 |
| Des_Local | 疫病 |