I would like to know about Ubuntu and virus/botnets, specifically how likely it is that it could get infected.
In terms of viruses, most of them spread so rampantly on Windows XP because almost everyone logs-in with administrative privileges. In Ubuntu (Linux), you should never log directly into the system as "root" (in fact Ubuntu doesn't let you log in as root, by default). In this way even if you did download something malicious, it would most-likely infect only the files that your particular user has access to. I've heard of viruses out there capable of finding their way around the Linux security model, but that's more the very, very rare exception rather than the rule.
For those rare times that you absolutely need elevated privileges, you can prefix those commands with "sudo" (while in terminal). But even then, you should exercise caution and take some extra time to think about what you're doing. Even a good security model doesn't save you from bad system administration.
In any case, I've run Fedora and Ubuntu at home for 6 or 7 years now and I've never had a problem with viruses.
Yes, it is possible but it can only happen if you either install software from sources that are not worth your trust or if you give away your passwords (and let someone else install software from sources nor worth your trust so basically it is 1 problem ;) ).
So if you keep yourself to Ubuntu Software Center and do not download random
zip files you are very safe. However, also be aware that some PPAs (personal package archives) could similarly be infected; the fact that they are installed by aptitude does not make them any less of a threat. Be careful if you read a blog and decide you want to add a PPA to your sources list without doing exactly the same level of thoroughness you would with a random tar or zip.
That is basically also Windows' problem: people download at random whatever they find on the web. So they are bound to run into programs that have been altered to mail passwords or other things to someone else.
Of course it is possible. However probability of such event is significantly smaller. Viruses are mostly created for Windows OS. Because of its popularity and fact that most of its users are no Geeks. I have installed Ubuntu on my PC at work. I am academic teacher and quite often students bring me their projects on USB drives. Due to that I had many issues with viruses on Windows. Since I switched to Ubuntu (about a year now) I hadn't noticed any problem with infections.