Imagine this. You are in charge of public health and must deal with an unrelenting epidemic. You have two options for protecting the population.
The first option is to monitor each person for symptoms of infection. You buy analytical technology and infrastructure, hire staff and build hospitals. You send forth specialists to monitor everyone. When they notice symptoms, more tests are performed. The symptoms are
subtle (fatigue, headache, stiffness), and healthy and sick people look a lot alike, so to be on the safe side you test far more people than are truly ill. Once you suspect infection, you quarantine the person and start a course of treatment. Sometimes the people are cured. Sometimes they are not. You can’t guarantee that you will find everyone who is infected. Or that everyone you treat is ill. The monitoring and mandatory quarantine intrude on civil liberties, disrupt lives and interfere with the economy. To compound matters, the disease mutates, so you have to continually design new screening tests and retrain the specialists.