does virus and worms affects linux ?

Hi there, i have some questions about virus and linux, can you help me ? 1 - I would like to know if linux is affected by windows virus and worms ? 2 - I would like to know it is possible to write and make a virus that somehow works in linux ? 3 - should i fell threatened by virus that possible exists in the internet ? 4 - how many virus exists and works in for the linux ? 5 - What kind of protection could i use (Anti-Virus software) ? 6 - Is linux somehow better developed in a way that is very difficult to write virus and worms? Thanks for your time Best Regards Marco Oliveira