subsequent giants in the field of virology included the dutch botanist martinus beijerinck , who in 1898 intuited the unique nature of the virus as `` a self-supporting molecule '' ; the american chemist wendell stanley , who subjected viruses to the indignity of crystallization , thus proving once and for all that they are not alive in any conventional sense ; and john enders , also an american , who in 1949 succeeded in growing the polio virus in his laboratory -- thereby making possible a vaccine .