Herd immunity is the fact that if you want to make an epidemic impossible, you don't need to have every member of a population be immune to the disease -- just a certain minimum percentage. What that percentage is depends on the pathogen and the animal.
It was discovered in the 1930s, when it was noticed that if a sufficient number of people got immunized for smallpox, the number of new infections decreased in the unimmunized population as well.
This is important because not everyone can get immunized against important diseases, making it more important (from a public health perspective) for people who can get immunized to do so.
For example, you can't immunize infants against pertussis, but pertussis is most catastrophic to infants. But vaccinating a sufficient number of adults against pertussis also provides protection for the infants.
"For me I'm mostly worried about losing my phone and some crook finding it, in that case fingerprint auth works well."
I suppose so, but it's pretty easy to lift a print and reproduce such that the scanner is fooled. Your print might be on the touch screen of the phone itself.
Personally, I find this an inadequate amount of security, considering the sensitive nature of the data that phones tend to accumulate. The odds may be low of a breach, but the consequences could be high. I'd prefer a slightly less convenient, but much more secure, method such as a long PIN.
But I do believe that the answer to "how secure should I be" is a very individual one, and so my preference isn't relevant to you.
I just worry that, particularly with fingerprint scanners, people tend to overestimate their security and might make different choices if they understood.