Question: Why can some diseases spread?

Ben Mulhearn answered on 16 Mar 2018:

Diseases spread to other people because they are contagious, meaning that, either through coughing, sneezing, or bodily contact, microbes can go from person to person. That’s how they survive. Within our own bodies, microbes can spread because of a few reasons. One is that they can sometimes hide from our own immune system (such as bacteria known as TB which can live within immune cells), or in the case of viruses, they can ‘replicate’ within our own cells which then burst out and infections many other cells, really quickly. Microbes can also spread through our bloodstream, or in our lymphatic system.