Patrick Brontë considered that his daughter Charlotte was not strong enough for marriage, and he sems to have been right at the time, before doctors and hospitals were able to cope properly with hyperemesis gravidarum, a kind of extreme version of morning sickness. This was probably what took her away on 31 March 1855. The catch-all 'phthisis' was written on her death certificate.
American member Paul Danigellis draws our attention to the following article in the Guardian newspaper: http://commentisfree.guardian.co.uk/ally_fogg/2008/01/the_bucket_stops_here.html
This is morning, afternoon, evening and night-time sickness and it blights somewhere around three in a thousand pregnancies. At its worst, the sufferer is unable to keep down so much as a sip of water, leading to severe dehydration and malnutrition. This is so debilitating that reading, watching TV or facing daylight may become unbearable.
If you've ever been hit by a bad oyster you can possibly sympathise, but imagine such food poisoning lasting not for a day or two but for 8, 16, even 36 weeks. In the days before IV drips, the condition was fatal for the likes of Charlotte Bronte, but now patients are mostly kept alive with regular inpatient stays and the magic of a saline bag.