Argemone mexicana called as Mexican prickly poppy is a species of poppy found in Mexico and now widely naturalized in many parts of the world with broad range of bioactivities including anthelmintic, cures lepsory, skin-diseases, inflammations and bilious fevers. Plant parts of A. mexicana were serially extracted with hexane, ethyl acetate, methanol and performed antiviral and immunostimulant screening against WSSV and Vibrio harveyi respectively. The control groups succumbed to death 100% within three days, whereas the mortality was significantly (P < 0.5) reduced to 17.43 and 7.11 in the ethyl acetate extracts of stem and root treated shrimp group respectively. The same trend was reflected in the immunostimulant screening also. Different diets were prepared by the concentrations of 100 (AD-1), 200 (AD-2), 300 (AD-3) and 400 (AD-4) mg kg−1 using A. mexicana stem and root ethyl acetate extracts and fed to Pacific white leg shrimp Litopenaeus vannamei weighed about 9.0 ± 0.5 g for 30 days. The control groups fed with the normal diets devoid of A. mexicana extracts. The antiviral screening results revealed that, the ethyl acetate extract of the stem and root were effectively suppressed the WSSV and it reflected in the lowest cumulative mortality of treated shrimps. After termination of feeding trials, group of shrimps from control and each experimental group were challenged with virulent WSSV by intramuscular (IM) injection and studied cumulative mortality, molecular diagnosis by quantitative real time PCR (qRT-PCR), biochemical, haematological and immunological parameters. Control group succumbed to 100% death within four days, whereas the survival was significantly (P < 0.001) increased to 30, 45, 75 and 79% in AD1, AD-2, AD-4 and AD-5 diets fed shrimp groups respectively. qRT PCR results with positive correlation analysis revealed that, the WSSV copies were gradually decreased when increasing the A. mexicana extracts in the diets. The highest concentrations (300 and 400 mg g−1) of A. mexicana extracts in the diets helped to reduce the protein level significantly (P < 0.05) after WSSV challenge. The diets AD-3 and AD-4 also helped to decrease the coagulation time of maximum 64–67% from control groups and maintained the normal level of total haemocyte, oxyhaemocyanin level after WSSV challenge. The proPO level was significantly increased (Column: F = 35.93; P ≤ 0.001 and Row: F = 37.14; P ≤ 0.001) in the AD1-AD-4 diet fed groups from the control diet fed groups. The lowest intra-agar lysozyme activity of 1.63 mm found in control diet fed group and the activity were significantly (P < 0.05) increased to 4.86, 7.89, 9.12 and 10.45 mm of zone of inhibition respectively in AD1 to AD4 diet fed groups.