It depends what you mean by ‘one can get’. Do you mean you, personally? Then the following don’t apply. If you mean a human, read on.
First up we have FOP, or Fibrodysplasia ossificans progressiva. It’s a genetic condition in which the gene that controls bone growth in a foetus never switches off. Soft tissue slowly metamorphoses into bone, locking joints into place, preventing any movement. It can lock your jaw so you are unable to speak, and your chest will become covered in bone, making breathing difficult and leaving you vulnerable to respiratory disease. Amazingly, the life expectancy is 40, but by this point you can expect to be totally rigid from the additional bone mass
A skeleton of a man with FOP
Any injury to soft tissue, the body heals with bone. Due to its rarity, doctors rarely diagnose it, instead ordering biopsies to discover a cancer or other cause. These then heal as bone. Surgery to remove bone just promotes further growth. And cut or scrape will likely cause an increase in ossification in the surrounding area. There is no cure.
Fatal familial insomnia is a disease who’s onset is usually around the age of 50, and progresses from mild insomnia to total inability to sleep. For up to 6 months, a person may become more and more confused, uncoordinated, and delirious as they lack any kind of sleep. Drugs to induce comas are unable to shut the mind down, so the person continues on. A progression similar to severe and rapid onset dementia follows, before the patient finally dies. It is related to a gene found only in 25 known families worldwide, and is unavoidable and incurable.
Epidermolysis Bullosa affects the skin. Specifically, it is caused by a defect in the gene that codes for proteins that hold the skin together. Layers of skin require very little force to break free from one another. Everyday activities such as putting on socks or washing can lead to loss of skin and painful, raw blisters. Over time, scar tissue from repeated skin damage can cause fingers and toes to fuse into disfigured stubs. The pain is constant and severe, as all positions can be uncomfortable to either lie or sit in. Such damaged skin leads to an vastly increased risk of infection, meaning that patients must be extremely careful about anything that could potentially infect them. It can be treated with pain medication and moisturising creams, but there is no cure.
Finally, I suggest Harlequin type ichthyosis. Another skin condition that causes the skin to crack into dry plates, leaving deep and bleeding fissures between them. Most children born with this condition die within day six or weeks, but it is possible to live longer. Those that do are consistently dehydrated due to their skin being unable to keep in fluids. They too can be expected to be in constant pain, and at risk of infection. Death in usually by adolescence for those that survive the critical first few weeks outside the womb. Again, no cure is known.
An illustration of a baby with ichthyosis.