Design a Contact-Tracing / Disease-Spread Tracking System

Problem

Design a system to track the spread of a disease through a contact network and answer exposure queries.

Requirements

Functional:

• Ingest contact events between people
• Query who was exposed to an infected person within N hops / time window
• Update infection status
• Alert exposed individuals

Non-functional:

• Millions of people, billions of contact edges
• Correctness and fault tolerance first-class
• Privacy-preserving

Discussion points

1. Graph model and storage (edges with timestamps)
2. Bounded BFS for exposure within hops/time
3. Scaling the graph (partitioning)
4. Fault tolerance and consistency
5. Privacy and access control