Accursed Witch
Creature — Human Shaman
Spells your opponents cast that target this creature cost 
less to cast.
When this creature dies, return it to the battlefield transformed under your control attached to target opponent.
Infectious Curse
.
Enchantment — Aura CurseEnchant player
Spells you cast that target enchanted player cost less to cast.
At the beginning of enchanted player's upkeep, that player loses 1 life and you gain 1 life.
Wesley Burt
Rulings
If an effect causes Accursed Witch to transform on the battlefield, rather than leaving it and returning transformed, then Infectious Curse doesn't become attached to any player and is put into your graveyard. This won't cause abilities that trigger when a creature leaves the battlefield to trigger.
(wotc • 2016-04-08)
Accursed Witch's first ability affects all spells cast by your opponents that target it, including Aura spells and spells that have additional targets. It doesn't affect abilities.
(wotc • 2016-04-08)
Infectious Curse's middle ability affects all spells you cast that target the enchanted player, including Aura spells and spells that have additional targets. It doesn't affect abilities.
(wotc • 2016-04-08)
Rulings
If an effect causes Accursed Witch to transform on the battlefield, rather than leaving it and returning transformed, then Infectious Curse doesn't become attached to any player and is put into your graveyard. This won't cause abilities that trigger when a creature leaves the battlefield to trigger.
(wotc • 2016-04-08)
Accursed Witch's first ability affects all spells cast by your opponents that target it, including Aura spells and spells that have additional targets. It doesn't affect abilities.
(wotc • 2016-04-08)
Infectious Curse's middle ability affects all spells you cast that target the enchanted player, including Aura spells and spells that have additional targets. It doesn't affect abilities.
(wotc • 2016-04-08)