The SF is like a common cold or flu. All Influenza ailments are caused by viruses. Viruses are very minute self replicating (with the aid of a living cell) infective/transfective agents containing the genetic material packed in a coat of protein or covering. The genetic material is either a DNA (Deoxyribosenucelic acid) or RNA (Ribose nucleic acid). The virus has to multiply like all cells multiply by duplicating. Viruses can duplicate or multiply by making several copies of its genetic material, each copy then has to be covered with the coat and this new baby virus will come out of the living cell to infect other cells and other organs and then other organisms etc. DNA replication happens in the host cell using the host mechinary. The DNA is a spiral molecule of genetic codes with complement binding of bases (three bases code for an aminoacid production) very much like the zipper. Only complement bases bind to other to produce specificity. When the DNA replicates the zipper unzips leaving open ends of bases ready to match with the complement bases. At this stage the DNA replicator enzyme collects the bases from the physiological soup and goes on fixing complement bases to the open zipper, thus making a complete complemeneted zipper from each half of the zipper that started in the begining. So two new zippers are formed which again unzip and the each unzipped open zipper produces its complement and four complete complemented zippers are made. Like this several copies of this zippers once formed, the codes that are hidden in the zipper in the form of bases will integrate into the host cell protein production mechanism there by instructing the host cell to produce the viral coat protein. Once the viral coat is produced the DNA (zipper) gets into the protein coat to form a baby virus. This is capable of infecting neighbouring cell, next organ and next organism. If the virus contains RNA instead of DNA, then the virus makes a DNA compelement to its RNA and then uses coding to produce the protein. The process of reverse transcription (RNA to DNA) is doen by a specific enzyme called reverse transcriptase. This process is prone for mistakes, i.e. while adding complement bases the enzyme may make mistakes and wrong base is added more often in RNA viruses than in DNA viruses. This wrong coding once in a while makes a new starin of the virus. That is specifically why the RNA viruses are difficult to handle both for vaccine production and also by the host as its antibody produced once may not become effective against a new strain. All the flu viruses are RNA viruses, so they change frequently. Though they have specific affinity to organism and cell types, the errors made and new starins formed may infect non-target organisms as it happened with swine flu virus entering human system.
It is said that the co-occurence of swin flu was noticed as early as 1918 and then the same viral was found in infections during 1980s in America. Since the pigs and the herdsmen or pig-keepers were in close association, there could had been a possibility of swine and human influneza viruses mixed in the swine and the exchange of genetic material took place and so the swine virus was capable of infecting humans.
Please see this video/tutorial link to understand the swine flu more clearly