# Anomaly detection on subset of categorical data

Imagine that I'm a health department, and I have a data stream that describes whether someone gets sick after eating food from a restaurant. Each data point in the data stream arrives 24 hours after eating has occurred, and has the following data elements:

1. Binary outcome: does the person get sick within 24 hours after eating at the restaurant?
2. restaurant_type (categorical variable such as Chinese, Mexican, Italian, etc)
3. restaurant_zip_code (categorical variable describing each location)
4. order_type (categorical variable which could be of "dining in", "take out", "delivery by restaurant", "delivery by uber eats", "delivery by doordash")
5. time_of_day (categorical variable describing time of day the order took place, which would be "6am-10am", "10am-2pm", "2pm-5pm", "5pm-10pm", "10pm-6am")

I want to create a model / algorithm that detects anomalies or unusual increases in sickness in the data stream. We expect that there would be some people randomly getting sick after eating food from a restaurant, so obviously we wouldn't want to investigate every instance of someone getting sick. But, let's say that there is a significant increase in sickness in people who "dined in" at Italian restaurants from a particular zip code from 2pm-5pm, then that would be unusual and I might want to investigate.

What is a good way to perform such a detection efficiently? One idea I thought of is I can generate many 1-D time series data based on each combination categorical values, and perform 1-D anomaly detection each. But, the number of combinations seems impractically large. For example, if there are 10 restaurant types, 40,000 zip codes, 5 order types, and 5 time_of_day ranges, that would be 10 * 40,000 * 5 * 5 = 10 million 1-D time series data that would need to be monitored.

Note: I'm not actually a health department. I am trying to efficiently perform anomaly detection on streaming data based on combinations of categories such as described by this similar "contrived" problem. Thank you!