Is the reason one bad bulb can take out an entire series of lights is that it breaks the circuit? Is that true of all light strands?
When mini string lights were first manufactured a string of lights would go out if one bulb burned out because breaking the filament in a single bulb would break the connection.
Fortunately, that’s not the case with most modern mini light strings.
In the early 70’s, manufacturers introduced a shunt wire at the base of each bulb. If you grab a magnifying glass and look really hard you might be able to see this little wire at the bottom of your bulbs. This engineering improvement means that if the bulb burns out (and the filament is broken) then the rest of the light strand will stay lit.
Keep in mind, that if you remove a mini light bulb or if its socket becomes damaged then the other lights in that circuit will go out.
Mini lights are constructed with one or more circuits. Depending on the number of circuits in your string of lights, only the circuit that contains the removed bulb will go dark.
For example, if your light string has 50 bulbs and they are all on one circuit they will all go out if a bulb is removed from its socket. If your set of 100 lights is constructed in 2 50-light circuits then half the total set will have an issue. This helps to localize your issue.
Most of the time when part of a string of mini lights goes out it’s an issue of either a failure of a shunt wire or a bulb is partially or completely unseated.
You can repair your set by replacing the missing bulb or by using a basic tester to diagnose your strand. A widely available tester is the Light Keeper Pro. You can see more info on this product at the manufacturer website: http://lightkeeperpro.com
We sell light keeper pros – you can see our price by clicking here.
Most of our discussion in this answer has been related to mini lights. You might also consider discussing Christmas lights that are C7 and C9 bulbs installed in stringer wire – those sets are generally wired in parallel – that means that you can remove a bulb from it’s socket and the rest of the strand stays lit.
That’s also why you can use traditional and LED bulbs in the same string when using this type of lighting.
(See these types of strings here: http://www.christmas-light-