Here's the problem, this issue is entirely due to you not knowing how his skill works, but still commenting upon it.
The solution is to look up the information yourself, that is freely available, and hosted in multiple places, including the very same website we are posting on right now, which also includes patch notes, patch histories, etc, so there are multiple ways on this site alone to figure this out.
If that still isn't enough, there are entire third party sites devoted entirely and only to giving out this information. For example, there is: http://leagueoflegends.wikia.com/wiki/League_of_Legends_Wiki Which has just about every single formula in the game except for their match making algorithms, including the damage and function of every character's abilities.
You could go here: https://na.leagueoflegends.com/en/game-info/champions/Twitch/, without even changing sites, and see that his E works like so:
Contaminate
Cost: 50/60/70/80/90 Mana
Range: 1200
Twitch wreaks further havoc on poisoned enemies with a blast of his vile diseases.
Deals 20/25/30/35/40 physical damage plus 15/20/25/30/35 (+20% Ability Power) (+0 [actually an error, should read .25 Bonus Attack Damage]) per stack of Deadly Venom to all nearby enemies affected by Deadly Venom."
Or you could go here: http://leagueoflegends.wikia.com/wiki/Twitch
And find this:
Contaminate EFFECT RADIUS: 1200
COST: 50 / 60 / 70 / 80 / 90 Mana
COOLDOWN: 12 / 11 / 10 / 9 / 8
Contaminate
Active: Twitch deals physical damage to all nearby enemies afflicted by Deadly Venom Deadly Venom, with each stack granting bonus damage.
Base Physical Damage:»
20 / 25 / 30 / 35 / 40
Minimum Damage:»
35 / 45 / 55 / 65 / 75 (+ 25% bonus AD) (+ 20% AP)
Bonus Damage Per Stack:
15 / 20 / 25 / 30 / 35 (+ 25% bonus AD) (+ 20% AP)
«Maximum Damage:
110 / 145 / 180 / 215 / 250 (+ 150% bonus AD) (+ 120% AP)
Contaminate cannot be activated if there is no nearby enemies afflicted by Deadly Venom Deadly Venom.
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Ability Details
Contaminate is a point-blank, auto-targeted ability.
Applies spell effects as an area of effect ability.
Soul Siphon.png Spell vamp is reduced to one-third effectiveness.
Rylai's Crystal Scepter item Rylai's Crystal Scepter will apply a 20% Slow icon slow for 1 second.
Banshee's Veil item.png Spell shields will negate the damage done by this ability.
Additional Information:
Deadly Venom Deadly Venom stacks remain on enemy targets after Twitch uses Contaminate.
Twitch is given a range indicator for Contaminate's radius upon infecting an enemy champion with Deadly Venom Deadly Venom (actual range is slightly larger than shown in-game)
Contaminate will not damage enemy targets whose Deadly Venom Deadly Venom stacks wear off or if they move out of Contaminate's range before Twitch can cast."
But here's the kicker, Twitch's E has functioned like this for years and years, he's been played in LCS, discussed quite in depth for years, and yet still people seem to have no idea how his kit functions, or what his skills do, or more specifically, what RIOT even changed, even when directly quoting patch notes, and then they make ludicrous assertions and claims like, "the base damage was halved at max ability rank." When it was decreased by 13.79% to justify reverting it half way, to being a less than 7% decrease in base damage, on one of the best scaling abilities in the entire game, that is also AoE, and on a typically overperforming and frustrating to play against ADC.
I'm sure you can see where the frustrations might arise from those who actually have a clue what they are talking about, and have to read this drivel day in and day out on the boards and reddit-- the only two places other than twitter where you can actually talk to RIOT.