This could maximize the effectiveness of the tests used. 

My 2 cents: For simplicity, this could use an existing network API (Facebook or Google) to connect to people who I was with and map how probable it is to be infected and to infect others without realizing it.

Might be a bit harder to achieve, but importing Location History data from Google Maps might be a great accelerator for people to get their last locations uploaded (and with higher definition).

Since this is sensitive data, it should be optional and with an opt-out (delete) feature.

There is no API to import Location History data, but any user may download it via Google Takeout and then upload to the app.

A similar idea in MIT (for inspiration) https://www.technologyreview.com/s/615372/coronavirus-infection-tests-app-pandemic-location-privacy/

I agree with this concept. 

This should look at location history based on the incubation period of (virus/disease). For Coronavirus that is 14 days, but you can add an additional 50% for security.

This should also link to social media like Facebook, etc. to allow the import of family and friends, especially those in a similar region since you will likely be interacting with them.

In a database, then your geolocation by time would need to be matched with other geolocations by time. Self-reporting would be a necessary step but could be accelerated with partnerships, media awareness, and adaptation by large companies for their employees.

Additionally, phone records could be pulled in to find those people by phone # that you have recently contacted, again checked by geolocation.

The Harvard app has major issues (read the reviews)

Good things to pick from them: Encrypted users.

Singapore's take at this seems interesting (even if their iOS app feels lacking):

TraceTogether (https://www.gov.sg/article/help-speed-up-contact-tracing-with-tracetogether) relies on bluetooth and shared use of the app to trace who you've been in contact with (or within contagion parameters, I guess?).

The information is kept private and local to your phone, until someone is tested positive for COVID-19. At that time the users will be asked to upload the collected info (identifiers of people also using the app that were near them while they were also using the app in the last 21 days), and anyone that was in contact with them will be contacted by health authorities to conduct contact tracing.

Contact tracing seems to have proven quite effective in Singapore, a heavily dense country that still has less than 500 cases two months after the first case was detected over two months ago.

App limitation on iOS: keeping bluetooth on in the background (in iOS only)  seems to be tricky (the creators have not been able to at least) - maybe some of our Mobile experts have a way around this?

I went to Mcdonald's and they gave me paper and pen to fill up personal information and this idea comes to my mind. 

+1