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As Reza describes, many hosts only have a single connection to network, so that single connection is always a possible point of failure. But there's a couple of things you can do to minimize the impact of a network infrastructure device failure.
When working with small appliance type switches, you might have multiple smaller switches rather than one large switch. For example, instead of having one 48 port switch, you might have two 24 ports switches, or six 8 port switches, etc.. If a switch fails, not all hosts lose connectivity.
You can also have additional ports, ideally enough to handle lost of any one unit of hardware, So, for example, if you have seven 8 port switches, when you only need 48 ports, if a switch fails, you only lose 1/7 of your hosts until they can be repatched into available ports on the other switches.
If some of your hosts have multiple NICs, then there are various method to use the two NICs to avoid a single network unit failure from dropping the host. Usually only shared servers merit that level of redundancy.
