Carriers of virulent strains spread polio, increasing the difficulty of poliomyelitis eradication. As rabbits became more resistant the viral strains responded by becoming less virulent. Canine parvovirus causes a virulent and contagious disease in dogs. Lozenges containing S. salivarius are marketed to support immunity against more virulent Streptococcus strains. This antigenic shift could cause the formation of a highly virulent virus. Thus, less virulent strains are selected. The fitness of the more virulent strains are increased by the vaccine. More velogenic (highly virulent) strains can cause caseated abscessation of the oropharynx. The CS also spread antisemitic prejudices, albeit never as virulent as the Nazis eventually became. There are proposals to remove the virulent genes from viruses to create vaccines. Genetic engineering could theoretically be used to create viruses with the virulent genes removed. Antisemitism was particularly virulent in Vichy France during World War II. The noun "virulence" derives from the adjective "virulent". He thought oxidizing bacteria made them less virulent. Of these poisonous plants, sumac is the most virulent.