No one really lives in isolation; the world is one big shared space. Whatever any human does inevitably affects those in close proximity to them. And one of the most significant effects of human behaviour is the impact on other people's health.
You must always remain acutely aware of how your seemingly innocuous behaviour affects the overall health status of others. An obvious habit to start with is smoking. Even the dumbest individual is now well aware of the hazards of cigarette smoking.