Hello!

So if you've never heard of rickets, congratulations on living in the 21st century. There's never been a healthier time to be alive 👍

So basically, rickets is more a set of symptoms than it is an actual disease, and those symptoms are caused when a growing body doesn't get enough vitamin D or calcium. That can be caused by either an underlying disease that messes with your body's ability to absorb vitamins and minerals, or by poor diet and lack of enough natural light.

In a culture where food isn't enriched with extra vitamins and minerals and adulteration of bread with things like alum is common, the Price family isn't going to have the most nutritious diet, and on top of that, poor Fanny as the eldest daughter would have been stuck inside all day every day helping out her mother. It's entirely possible that her sunday walk to church was the only time she got any direct sunlight.

And now that I've gotten all that out of the way, what are the actual symptoms of rickets? The main ones are stunted growth, bone and muscle pain, and weakness.

Fanny is noted in the first chapter to be very small for her age, and her fatigue and issues with things like short walks are noted repeatedly. Of course a lot of things can cause symptoms like that and that poor girl has Chronic Illness(tm) written all over her, but I really do think rickets makes sense for one reason. She gets better when she can spend a lot of time outside and worse when she's confined to the house. More than a few days without riding or otherwise being outside are shown to lead to worsening fatigue and weakness.

Why? Because time in the sun causes the body to produce more vitamin D than would be the case otherwise.

In the present day, rickets is a fully treatable condition, usually only requiring vitamins to reverse all the symptoms (sometimes surgery to correct bone deformities, but many of those will resolve themselves over time). But Fanny doesn't live in the present day and she doesn't have a doctor making sure she gets her 2,000 IU of D3 every day. So while moving to Mansfield (and getting a relatively healthy diet for a change) would absolutely have helped her symptoms, but she was probably always struggling with a chronic low-grade case of rickets that got better or worse depending on how much time she was able to spend in the sun. (Both the diet and sunlight things would also explain why going back to Portsmouth made her ill again.) Maybe Mr Crawford wasn't entirely full of shit when he noticed she had a growth spurt at some point after her cousin's marriage. Maybe that was her body beginning to self-correct after years of better nutrition and more frequent exposure to the sun.