You don't have to worry about the computer having viruses because Tails runs independently from the other operating system and never uses the hard disk.But, Tails cannot always protect you if you install it from a computer with viruses or if you use it on a computer with malicious hardware, like keyloggers.
[[!img amnesia.png link="no" class="svg" alt=""]] <i class="metaphor">Like a tent, Tails is amnesic: it always starts empty and leaves no trace when you leave.</i>
[[!img persistent-storage.png link="no" class="svg" alt=""]] <i class="metaphor">Like a backpack, you can store your personal things in your Persistent Storage and use them in your tent.</i>
You can <strong>save some of your files and configuration</strong> in an encrypted Persistent Storage on the USB stick: your documents, your browser bookmarks, your emails, and even some additional software.