So security updates solve all security problems
No it does not.
Paranoia is good, defense in depth is even better
Sticky on security
It will all depend on the time you have, the degree of risk you can take,
the cost / benefit analysis.
Do backups, have a good firewall, tighten security, do update,
remove unecessary services, audit your system, keep an eye on your logs,
layer the security, etc.
Among good stuffs: snort, nessus, tripwire, clamav, SELinux etc.
But you do not want an overkill of work...
how viruses get in to linux systems?I read that new viruses appeared to run in both windows and linux
Well, read the exact news about that, it was just one proof of concept,
and the severity of which is debatable
So this was the serious answer.
The less serious answer is yes linux is pretty secure by design,
some distro give or implement more or less security by default,
and you can be as paranoid as you want.