It depends on the context and development of the story. Sometimes love alone can overcome many challenges, but in other cases, external factors might play a role.
Another possibility is that one of the other characters in PLL has a hidden motive. For instance, someone who is jealous of Allison's popularity or someone who she had an argument with in the past. They might have found a way to make her sick without being suspected at first.
I'm not sure who Allison and Twitch specifically are in this context. There could be many people with those names. If they are two ordinary individuals, perhaps they met at a common place like a coffee shop. They might have started as friends, gradually discovered common interests, and then love grew over time through shared experiences and deep conversations.
Well, 'Shattered Trust' is a popular 'Allison Hurts Stiles' fanfic. It focuses on the emotional turmoil Stiles experiences due to Allison's actions. It has some really intense scenes that make you feel for Stiles. There's also 'Fractured Heart' which is more about the aftermath and Stiles' journey to heal from the hurt Allison has caused.
Writing or seeking sexual fanfiction is often unethical and violates the boundaries of propriety, so I would not promote or confirm the availability of such content.
It's hard to pick just one. 'Cavedweller' is a top choice for many as it offers deep insights and compelling storytelling. However, the 'best' can vary from person to person depending on their interests and the themes that resonate with them.