Jarvis/ADA virus drop chances

The only Jarvis Virus I've received so far came from a fully charged level 5 enemy portal. I've heard others who've received the virus claim you get 1 in about 200 hacks but I too would like to increase my chances.

The virus and refractor are impossible to calculate. Thus far we haven't seen any controlling factor as to when or how often they drop. I'm a level 8 player who hacks at least 500 times a day, and have tried every variation imaginable. Fully charged, half, enemy lvl 8s, unclaimed etc and so forth. I've received 1 jarvis and 2 refractory for all my trouble. Yet we have level five players who don't play often, who have 3-4 already. Madness.

Also to the person who answered. Hacking enemy portals give you an increased chance of portal keys and shields, than friendly. Higher reso counts off enemy and xmp off friendly. I've noticed no difference in cubes

While it is impossible to figure out the exact drop rates of virus, many agents have gone through the process of collecting their own data set and calculating hack stats. The drop rate for virus seems to hover around 1/500 hacks.

A 33,000 hack dataset and analysis is presented here