Spinal Muscular Atrophy
What is SMA?
Spinal Muscular Atrophy is an autosomal recessive neuromuscular disease (2 genes inherited through both parents are needed in order to have the disease). It is caused by defects in the Survival Motor Neuron 1 (SMN1) gene that codes for a protein called SMN. The SMN protein is critical to the health and survival of the nerve cells (motor neurons ) in the spinal cord, which are responsible for muscle contraction. Most people have two SMN1 genes, one inherited from their mother, and one from their father. In children with SMA, both genes are defective. 
In a healthy person, this gene produces a protein that is critical to the function of the nerves that control our muscles. Without it, those nerve cells cannot properly function and eventually die, leading to debilitating and often fatal muscle weakness. There are four primary types of SMA - 1, 2, 3, and 4 - based on age of onset and highest physical milestone achieved. 
Type 1 : Werdnig-Hoffman Disease (This is the type Lilah has)
The diagnosis of SMA type 1 is made when the child is between 3-6 months of age. This is the most severe form of SMA. A child with type 1 is typically never able to lift his/her head or accomplish the normal gross and fine motor skills expected early in infancy. They generally have poor head control, and may not kick their legs as vigorously as they should. They typically never bear weight on their legs or sit unsupported. Swallowing and feeding may be difficult and are usually affected at some point, and the child may show some difficulties managing their own secretions. Tremors can be seen on the tongue. There is weakness of the intercostal muscles (the muscles between the ribs) that help expand the chest when breathing. The chest is often smaller than usual, giving the trunk a bell shape. A child with SMA type 1 generally uses the diaphragm to breath, giving them the appearance to breathe with their stomach. Due to this type of breathing, the lungs never fully develop, and the child will have a weak cough. They may have a difficult time taking a deep breath while sleeping, affecting their ability to maintain normal oxygen and carbon dioxide levels. 
To read about SMA type 2, 3, and 4 view source  at the bottom of this page.
A more scientific explanation of how SMA works:
Motor neurons (nerves) are a wiring system which carry electrical impulses from the brain to the muscles, controlling how they contract and relax. Part of this wiring system is damaged or lost in SMA due to low levels of the Survival Motor Neuron (SMN). This SMN protein is found in the cytoplasm and in the muscles of all cells, where it is involved in the production of a complex molecule that is crucial for the formation of new proteins within the cell. This very basic 'housekeeping' function is essential for the viability of every cell in the body. However, SMN has also been linked to other chemical functions. 
Two genes code for the SMN protein: SMN1 and SMN2. We all have them and both play a central role in SMA. They are almost identical but, because of a small difference, only 10% of the protein produced by the SMN2 gene is functional. SMA patients, because of the loss of the SMN1 gene, rely purely on the protein produced by SMN2. Most cells can function normally with low SMN protein levels however, motor neurons cannot and they progressively die. This is why individuals missing or possessing a mutation in their SMN1 gene develop SMA. 
Although the other gene, SMN2 cannot fully compensate for the loss of SMN1 function, the levels of protein expressed from the SMN2 gene control the severity of the disease depending on the number of copies. This varies between individuals (Lilah has no copies of SMN1 and only 2 copies of SMN2). The more SMN2 copies a patient has, the less severe the disease. SMN2 provides an attractive target for developing SMA therapeutics and the majority of drug development efforts in the field are focused on increasing SMN protein production from this gene. 
SMA affects approximately 1 in 10,000 babies, and about 1 in every 50 Americans is a genetic carrier. SMA can affect any race or gender. 
SMA is a genetic disease where both parents are carriers. 2 carrier parents have a 25% percent chance of parenting a child with SMA, a 50% chance of parenting a child that is a carrier of SMA, and a 25% chance of parenting a child that is not a carrier and also does not have the disease.