Water reservoirs and other large water supplies are difficult to contaminate due to the effect of dilution on the concentration of toxins or microorganisms. In addition, standard water treatment methods of filtration and chlorination inactivate most viruses, bacteria, and toxins. However, several pathogens and toxins can survive standard water treatment; an example is Cryptosporidium parvum. Water could be used as a vehicle for biological attack if water was contaminated near the end-user.