The official and medical name for people with a fear of strangers is Xenophobia. This not only covers people who have a fear of strangers, but also for people afraid of foreigners and other people different to themselves. Xeno means a guest or stranger and phobia means to have a fear.
The meaning of Xenophobia is not only exclusive to people afraid of others different to themselves, but can also include fear of people from different backgrounds, with different attitudes and high confidence levels. It is also well known that people speaking in a foreign language can also be very threatening for those suffering with the illness.
Xenophobia is more a state of mind than a decision that the sufferer has decided to make. It is hard coded into their brains and no matter how much they are told that the person they fear is okay to be around, they will never believe this and argue this fact right to the very hilt and will never give in.
There are many Xenophobics who not only suffer from this mentally, but can be vocal and act violently towards the people that they fear. They can lash out and cause harm due to their extreme fear. Most sufferers however are placid and do not expose their fears to other people and keep it only to themselves.
It is extremely common in tribes across the world for this fear to still show strongly. Tribes can fight to the death if they find each other too different.
Hitler is the most famous person to have fought and killed over this fear. It is thought that he genuinely was scared of people different to himself and in retaliation had them all killed. This is probably one of the very most extreme cases of such issues.