twitter knows, when you are sick

Do you think, it is not possible?
Remember, when someone is going to be sick, he - or she feels anchy, tired ... a little bit a bit off. Adam Sadilek (and colleagues from computer science ) used twitter to predict when people are get sick. Precisely, they know it 8 days before they fall ill. They use machine learning and natural language understanding techniques. Learning algorithm used more than 630 000 twitter users with known gps position in NY city in one month. Their algorithm recognize people who talks about being "sick". After 8-days, 90% of people get really sick.
This is their results visualized on map. 

From:
Fast Company: Twitter Knows When You’ll Get Sick Before You Do

What do you think about Twitter privacy? I don't think they asked people, if they could use their tweets...