Let's look at the third method, which is secure MPC. Imagine Jane wants to know how many phones are reporting fever. John sending “one” means he has a fever and “zero” means he does not. To do this, John’s phone sends “70” to server P on the left and the number “minus 69'' to server Q on the right. Of course, 70 minus 69 is one. The second phone sends 38 and -37. Third sends 42 and -42. The Left server adds up to 150, and the right server to -148. And that leads to the answer 2 people with fever which Jane sees without knowing about John or his fever status.
The goal behind using secure MPC in our toolkit is to facilitate anonymous symptoms reporting by the public, without compromising their individual privacy.