During a pandemic, we think about our immune systems more than usual. One of the factors that may determine how easy it is to catch a virus is that particular virus’s “infectious dose”: how much of the particle must enter your body before an infection is established. Why does a dose lower than this threshold not make you sick? Because your immune system can destroy the viral particles before the infection takes hold. And of course, when you do get sick with a virus, and then after some time get better again, it’s because your immune system has succeeded in killing the virus that caused the infection.