No, a virus is not a multicellular organism. It isn't even alive by current definition. These little critters are formed of DNA or RNA, which are nucleotide chains. They're some 100 times smaller...
Virus is considered as a living organism since it doesn't have what we call a well-defined cell. It has a protein coat and genetic material only. It is a matter of controversy whether to classify...
Yup. In the general sense of the term "virus" (any malware that can be installed on a computer), there are some kinds of malware, usually called downloaders, which can actually download more malware...
Use a good antivirus to know if you have a virus. Examples are: Avira uit Duitsland avast! uit Tsjechië BitDefender uit Roemenië Computer Associates uit de VS ClamAV - GPL ClamWin - GPL ClamAV voor...