The title is a little provocative (again), but the antivirus can actually have negative sides.
Let's start with the benefits of antivirus:
The antivirus starts as soon as you start your computer, it starts before other programs through "filter drivers" which choose the boot order. For example it allows the start of keyboard and mouse processes before others.
It protects your PC from the start until you turn it off
How does it protect your PC?
Antivirus can scan files on your computer when they are created, renamed, executed or when you want to launch a global scan. It creates and compare a unique signature to each files with its database containing infected files signatures.
You understand it if the signature is in the database it launches the alert.
From there we know that the files do not escape the antivirus and it remains only to hope that it contains a maximum of signatures in its database.
One can also scan a file directly with several antiviruses on VirusTotal
Antiviruses also use other methods such as heuristic detection: to determine suspicious signatures without having them in the database. Thus if a program tries to delete files from the system or replicates itself 100 times it is considered suspicious and the antivirus launches the alert.
I quickly pass the other benefits of antivirus such as integrated firewal, real-time web scans, secure runtime environment etc ... In short you are protected with an antivirus and an update firewall. It is absolutely necessary to install one. It's like going out in the snow at -15 ° C all naked so do not worry about getting sick afterwards. Antivirus plays the role of the coat and drugs of last resort.
Why don’t they protect you 100%?
You should know that is possible to make a program undetectable while it was before.
From that moment on the whole part on the scans collapses. Indeed it’s possible to modify a part of the source or binary code of a program to change its signature without changing its behavior. There are even programs called Crypters which allow this code to change automatically to make it undetectable.
The heuristic detection part also has car limits: you can pass off a malicious program as a legitimate program if you rename it, change the icon (by copying an icon of an existing program) etc ... Moreover the opposite direction may arrive, it’s possible that a legitimate program is detected as suspicious. Therefore the antivirus publisher must properly set this type of detection. Whatever it is, it's easier to pass of a suspicious program as a legitimate one. And so the heuristic part collapses...
The famous programs POTENTIALLY undesirable
This is a big point that I want to mention now.
Too much antivirus software, antispyware, etc, have decided to detect a maximum of "problems" (by detecting them by "mistake") attempting to frighten users off to make them pay for a "premium" version which they don’t need. Far from me the idea to blame an individual software in particular or to affirm that absolutely all the problems detected are wrong, but there are still limits ...
Saying for example (real case) that a program is an adware while he doesn’t have any publicity is gross...
Also say (other real box) that program is a Trojan while the program is healthy and published from Microsoft is even more gross.
In other words, the antivirus (in general) is sometimes wrong (often?) and it’s appropriate for everyone to properly study the scan results.
The reason for these "overdetections" is simple: the user naturally goes to the antivirus that "finds the most virus" without thinking about the quality and accuracy of the results.
So we’ve seen that an antivirus is essential because it stops the classic threats. That said do not give blind trust to your antivirus because someone who specifically targets you will go through the net of the antivirus even if you naively believe to be protected.
This is why antivirus is not your friend at least not really because they shouldn’t make you reduce your attention. Never.