There are many health conditions that can cause delayed development. You could have a child that has asthma, making it hard for them to breathe and hard for them to have endurance while playing, which could cause them to have delayed development.
You could have a child that has chronic illness. You could have a child that has even a tendency to have a lot of ear infections or even a child that has enlarged adenoids or tonsils could also impair their speech which could then delay their development. Another really important thing to note is any child who has chronic illness or any child who goes to the doctor's office and they're not in school learning and playing can cause delayed development simply because you're removing the child from their opportunities for them to learn, so that would make it very hard. A great thing to remember or to note is if you go away on vacation from work, and you come back and your mailbox is full and then you're trying to play catch up.
Well anytime a child is not in school or in their normal environment where they're playing and learning that's how it is for them. Except that it doesn't just take them a week to catch up, it can take them a lot longer. So anytime that a child has chronic illnesses or chronic visits to the doctor's, and away from their primary place of learning can cause a delay in their development.