Answer:
Virus enters into the host cell through a process called endocytosis. 
Explanation: 
Endocytosis is a process in which molecule enters into the cell through cell membrane. Virus also enter into the host cell through this process. After entering, the virus starts replication and grow in numbers inside the host cell and take nutrients from it which is used in the process of reproduction. When virus population increases, it cause infection to the neighboring cells. 
 
        
             
        
        
        
Respiratory system, because the lungs and diaphragm would get the oxygen from the air, and transfer them to the circulatory system (that's a pretty basic explanation, there's much more to it)
        
                    
             
        
        
        
Answer:
Number of protons plus the number of neutrons. 
Explanation:
Most of an atom's mass is in its nucleus. The nucleus is made up of protons and neutrons.