Case clustering: a. is of indeterminate significance for rare diseases, because clusters may occur by chance alone b. suggests common exposure of a group of people to an etiologic agent c. refers to spatial distribution of disease d. refers to temporal distribution of disease e. all of the above are correct

Which of the following represents a difference between viruses and viroids? a) viruses infect many types of cells, whereas viroids infect only prokaryotic cells.b) viruses have capsids composed of protein, whereas viroids have no capsids.c) viruses have genomes composed of rna, whereas viroids have genomes composed of dna.d) viruses cannot pass through plasmodesmata, whereas viroids can.