Ebola hemorrhagic fever is caused by the Ebola virus of the Filoviridae family of viruses together with he other nasty Marbug virus that also causes hemorrhagic fever. It is transmitted to human from infected animals like bats and monkeys. Ebola causes hemorrhagic disease whose symptoms are fever, vomiting, diarrhoea, rash and eventually internal and external bleeding. People can get infected if they come into contact with sick ebola patients or infected material through broken skin or mucous membranes (e.g. eyes). Luckily Ebola outbreaks have been few and rare and end quickly. The current outbreak has been the longest and has affected the most people. There are 2 potential vaccines that are being tested. Fingers crossed!
There are many nasty viruses that cause deadly disease. An example is Rabies. Luckily there is a vaccine and treatment. If you get bitten by a dog or cat or bat make sure to see a doctor as soon as possible.
Priscilla has given a very good answer to this. However, as a virologist, I felt I had to chip in too.
Our bodies clear pathogens (viruses, bacteria, etc) through the immune system. The immune system detects pathogens/cells through proteins or carbohydrates on their surfaces. Therefore, the immune system can distinguish between what is good for the body/part of the body and what is bad (foreign). If it is good, then the immune system does nothing. If it is bad then it is killed but the immune system remembers it so that if it comes again it will be killed.
Then how does disease come about if the immune system can do all these? Why is ebola killing very many people?
What makes viruses (such as ebola) hard to control is because they change very fast such that sometimes the immune system fails to recognize them and they proceed to cause disease. The current ebola virus is different from the previous ones. Detecting it in the lab is a problem, so a person can have ebola virus but when we do experiments in the lab we get negative results. SO be careful when you meet a person with symptoms Priscilla listed.