Antivirus for Ubuntu 18.04

This article suggests not to, but if nonetheless you wish to install AV software, ClamTK downloadable from the Ubuntu Software Centre will provide some peace of mind.

Alternatively as described here you can open a terminal and type:

sudo apt-get install clamtk

or load it through Synaptic Package Manager.

Based on my experience from using Ubuntu as my main operating system for more than 5 years and 18.04 for more than one year, you do not need anti-virus software at all. I don't use it and never had any problems so far. That said, you should always adhere to the usual safety rules: Do not open/download unknown files, install software from trusted sources, keep your system updated, etc....

EDIT: See this article for further information.

Anti-virus software does not just look for viruses.

You will also want to look at rkhunter which looks for root kits.

While Linux/Unix has little notion of viruses, that does not mean to say that software cannot be loaded on to these systems through known vulnerabilities and be executed. Anti-virus software can also look for these.