Mavis Pike used to have another child, who died.
- Either before Frank was born, or when Frank was still a baby/very young. A lot of Mavis' quite frankly neurotic levels of over-protectiveness could be easily explained as a result of trauma at losing a young child, and terror of it happening again. Most likely the first child died of some kind of infection or illness, hence Mavis Pike's constant obsession with making sure Pike's wrapped up warm to protect his chest, and her horror at Pike doing or touching anything that she feels is covered in germs, like a public telephone. If Frank was still alive when the first child died it could also explain some of her babying of him, as she wouldn't want him to go through any more trauma than he already had.
Alternatively, it was Frank Pike himself who got sick as a young child and almost died.
- Whatever happened, he got very very sick, to the point where no-one expected him to survive, and when he did, Mavis and Wilson were so relieved to have him back they spoiled him and babied him, while Mavis also became extremely overprotective, so that it would never happen again.