Human beings are flawed. So no system of defense that depends on human action is going to be able to protect us 100% of the time. But we can prevent most problems. In 1990, Professor James Reason came up with the "Swiss Cheese model" to create defenses that are hard to break through.
Think about Swiss cheese—it has lots of holes in it, right? Well, the Swiss Cheese defense model takes the "holes", or human flaws, into account by using several layers of defense. Each layer has holes, but not every layer has the same holes. So, put enough different layers together and there won't be a complete series of holes that line up to allow something through. Each human failure will be blocked by other successes.
Scientists and public health specialists are using this model to protect us from the novel coronavirus with layers of protective practices like wearing masks, washing our hands, staying home, etc. As we know, not everyone follows all those measures. But if enough people do, the holes will be smaller and more layers will be in place to block them. We'll be able to slow the rate of infections—and deaths—dramatically.