The Covid-19 Virus or Coronavirus was spread to Bob via another person who was not wearing a mask who happened to have the virus. The Pathogen entered through the nose and it has started to make its way to the lungs.
The first line of defense tries to prevent the virus from entering fullys with some of the physical or chemical barries avialiable like mucus, tears, Cillia and Gastric Acid this is a non-specific defense system.
The Coronavirus pathogen has made it past the first line of defence and been recognised as a foreign cell/agent (non-specific) and the secound and third line of defense start taking action
The B-Cells (Lymphocyte) try to identify this pathogen and its antigens that surround it, it takes some time but the correct antibodies were found to attch to those atigens and attack the pathogen.
The B-cells with the specific antibodies to fight the pathogen are duplicated and these antibodies are duplicated many times to help attach to the atigens and kill the pathogens.
The pathogen is forced out of the body with the dead blood cells, some of the B-cells are kept as memory cells with the specific antibodies that will be used if the pathogen is encountered agian and it will be recognised and attacked quicker if it makes it into the body creating an immunity agianst this specific virus.