The reward system in the brain is the center of wanting and pleasure, and is the main cause of repeated actions because of the release of some neurotransmitters which does ensure the survival of one's self and offspring. The behavior is a part of organism and organizational level, and neurotransmiters are part of the cellular level.
One is probably because your friend has a stronger immune system than you and is able t fight the bacteria infection. The other is probably that your friend is also infected but does not show any symptoms of the infection, in what is referred to as latent infection. The bacteria enters a lysogenic cycle in your friend’s body rather than the more virulent lytic cycle (that happens to you).
Good social distancing would be staying at home but if it’s a must to go outside. Staying at least 6 ft and so that way we don’t all die♀️