The simple and short answer is that no human or animal cells have "Cell Walls." These structures are unique to plants, fungi, and bacteria. Drugs that fight bacteria, viruses, and fungi rely on finding some difference between our cells and theirs, then sabotaging something they need to happen but we do not.
Imagine each cell is like a water balloon. Animal cells must move to eat and never developed a more rigid structure. Plants, Bacteria, and Fungi evolved the Cell Wall, which is like a cage of complex sugars they make around themselves.
After they had one, they further evolved to depend upon it. They often exist in environments with a much higher water consent than their insides. Water would like to diffuse inside, like having the balloon hooked up to a hose. Without the wall, the bacterial cell blows up until the balloon pops. The cell wall acts as a cage around the balloon, Keeping it from swelling up enough to pop.
Penicillins prevents certain bacteria from using the little tools they need to make more cell wall. Imagine a little robot made to build a house, put materials in one end and house comes out the other, Penicillin breaks the part that makes nails. The robot can;t adapt to that, so it keeps trying to build a house with no nails, which falls apart.
Now there is no new Cell Wall, but the cells don't know that. The cells inside continue their normal growth, but are stuck with the cage they have. Eventually they outgrow the cage, breaking it, At which point they pop like the aforementioned water balloons.
Animal an human cells have no cell walls. Unless thy have evolved other mechanisms to compensate for water trying to rush into their balloon, they have to stay in areas where the salinity is just about that of the ocean.
Large animals like us developed a way to bring the ocean with us. Even though your skin may seem dry, it is a barrier of dead cells. The living cells are surrounded by and "internal sea" of blood and interstitial fluid; this with salts and proteins so that the effect is that there is as much junk outside the cells as there is inside. (Osmolarity is the technical term for how much junk is in a liquid) If this is severely disrupted in either way, the human cells will shrivel or pop and the person will get sick or even die.
