Is obesity contagious?

Abm4n

As a middle aged man I have found that I can lose 1 kg (2 pounds +) each week if I stick to a healthy and controlled diet but when I go around to my friend's house for dinner I put on 1.5 kg after eating his home made pizza and ice cream. He also eats far more than me so I am being moderate here.

All of my friends seem to be about 150% of my weight i.e. they are half my weight overweight and 90% of them have massive bellies. I have started to hang out with one friend who lives close by and we go walking in the morning but he can only walk a fraction of what I can in terms of speed and distance. What is the right thing to do?