Let us assume that there is a perfect antivirus program. It examines a program and says "Yes" if the program will infect our computer. It says $"$No$"$ if the program will not infect our computer.

Antivirus $($Program$)$:

If Program causes infection

Report "Yes"

Else

Report $"$No$"$

If such a program existed, we could write a program Evil that accepts a program as its input and asks Antivirus to look at it$.$ If Antivirus says "Yes", Evil stops without doing anything else. If Antivirus says $"$No$"$ about the program, Evil infects the computer:

Evil $($Program$)$:

If Antivirus$($Program$)$ says "Yes"

Exit

Else

Infect computer

Make an argument for how this program Evil shows that the program Antivirus cannot be written.