i have not yet heard of a virus for linux.
But you are still vulnerable to phishing,
getting conned out of your password by fake sites etc,
and perhaps javascript could do nasty things.

are there viruses for macs?

Will logging into root and invoking a virus is a sure way to lunch it. Mainly
you got to invoke the virus to get to run. However, there many different kernels
of Linux. It would be very difficult to get a virus to attack if at all.
Depending on the distro of Linux. The average kernel updates is every six
months. To get a Linux virus to attack the Linux kernel. The Virus has to go
through lots of hall ways, doors, bedrooms, bathrooms, kitchen, and still can't
find the source of the family room.
Not like Windows. Soon as the virus busted down the door, it is at the kernel,
and the virus is running amok with the families daughter.
Basically Linux is compiled with some many lines of code. Then you add a
desktop environment like KDE or Gnome. Then you add programs layered on top of
the kernel like GNU, Python, C++, BASIC, and etc. Hacking into a Linux computer
can be just as difficult.

There are a couple of reasons to use AV software in Linux. One is to
protect yourself just in case and the other is to protect others. You can
pass on viruses even if you can't become infected.

That being said, I do not use any AV protection. Linux AV programmes do not
work as well as Windows ones and are difficult to set up. Scanning email
attachments and downloads in the background as in Windows is hard to
achieve. You usually have to do this manually and it becomes more of a pain
than anything. After trying it you will find that it just is not worth the
trouble due to the remote possibility of infection.

Unless you are paranoid about security then you probably will not go to all
of that hassle. The reasons why Linux AV software is not as good is that
there is little need or demand and part of the reason for that is that
security is built into Linux. This is partly why it is hard to set up
scanning automatically. You need to give permission for anything remotely
risky, even AV scanning, since you will likely be scanning the boot sector
and the kernel.

K/Ubuntu has done away with much of this risk by not allowing root login.
Some distros allow you to do this and Windows users may be tempted because
it makes life easier to only input a password at login or not at all. That
is not the way Linux is meant to work and it is why Linux beats the pants
off Windows and OS X every year in an annual hacking contest. Linux security
when properly followed is good.

Many people speculate that Linux is not targeted because it has so few users
and that Widnows is targeted because it is so many. However, the challenge
of infecting Linux should be great enough to encourage virus developers.
Windows should be no contest for them. When Ubisoft said its DRM was hacker
proof, hackers broke it in a day. The challenge is out there. Linux has much
of the server market so maximum damage to big companies could be done if it
was possible.

The big difference I think is that Linux users are sufficiently wise that
viruses would not propagate. In other words, careless people get infected in
any OS and Windows encourages users to be careless. Linux encourages people
to take security seriously and therefore while some individuals may get
infected it isn't likely to spread far. Windows has tried to close security
holes, but Microsoft is so slow at it that a virus can make serious damage
before the react. In contrast, Linux is a collaborative effort and fixes are
often out the same day that they are discovered. Also most users update
often enough and are better informed about security threats that they
install updates right away. The thing that can hurt Linux the most is
Windows users who move over and bring bad Windows habits with them. That is
why I accepted sudo and no root login as the better method in the end. It
removes temptation.

As long as you follow established protocols I think that you are reasonably
safe and that if a threat surfaces it would travel so slowly that you would
have advance warning and can take further steps. However, there are Linux
versions of many popular AV applications such as Bitdefender and AVG. Use
them if you want. Chances are that the novelty will wear off.

Viruses are generally written to attack specific platforms. So, a Windows virus
generally cannot attack a Linux-based PC.

What follows is my personal opinion:

Because Linux makes up such a small share of the over-all PC population, it's
not worth the virus writers' time to attack Linux. Same thing applies for
Apple. What the virus writers are after is notoriety and bragging rights. The
best way to ensure that is to attack the biggest target. Sure, in some cases
the virus writers are intent on causing damage to a specific target group but
that's by far a minority of cases.

A second reason is that most Linux users are more computer literate than the
bulk of Windows users and so are less likely to click on the wrong button to
allow bad thing to happen. After all, the vast majority of viral attacks on PCs
require some level of user action for anything to happen.

You are talking desktop computers only. Linux and open source is a big
player in the server market. If virus writers were able to exploit Linux
then they could cause much havoc in some of the world's biggest corporations
and this would get them the attention they want. So, why aren't they?

Servers are targets for hackers as opposed to virus writers. If you're running
a good, secure shop, you won't do anything (browse the web from the server GUI,
for example) that'll provide a path for viruses to get directly at your server.
Desktops, OTOH, are devices that, by their very nature, are ripe for attacking
with viruses.

I understand that viruses cannot install themselves under Linux because under
Linux a root password is needed for installing programs (I have come accross
some exceptions, though, like a version of KBasic). And I came accross other
exceptions, as well.
I also understand that another thing is that, unlike under Windows, a virus
cannot enter your PC from the Internet or from the local network automatically,
you would have to download it and then run the virus that you desire so much.
And then the surprise might be that it is not compatible with your OS.
Am I right ?
I am sure that when Linux becomes more widespread, lots of trojans will be
written for Linux, also, but these will exploit the vulnerability of the user
itself, not the vulnerability of the system. There will be clever ways of
convincing careless users to run certain programs.
As for being infected automatically and in large numbers, with malware
produced and copied in industrial amounts like today, this will be simply
impossible, like cheerfully swimming in molten iron.

This is why it is recommended you stick with the official repositories.
These applications are used by many experienced users and should a virus
be introduced many would know real quickly and the problem fixed not
covered over.

How much easer would it be to write a virus, than if the source code was
given to you? Oh wait, I guess it is. Then why has it been so hard for
the sub-world to destroy Linux's rep as being hard to infect? Because
some of the best minds in the world work behind the image to keep it
solid, as well as security is built into the system.

I look at computers as portable safes with Linux having the dead bolts
built into the structure. I see MS Windows as a safe with the dead bolts
exposed on the outside of the box, with a simple bicycle lock keeping
out the honest people. The lock for MS is purchased from outside
companies like AVG and other large names, that have become famous
because Microsoft has no clue how to build in security.

Looking at the two safes side by side, even the casual bystander can see
the holes in one of the systems. Any one that believes that virus
programmers don't work on Linux viruses because there is not enough
market. Have bought the bill of goods the marketers of MS products have
sold them, and are still wearing blinders. Now both safes are only
going to keep the goods safe if you leave the door closed and locked.
For Linux this is done by using safe practices and safe repositories.
For MS Windows, well not so much!

No safe is secure if you don't keep it in your possession.