122 people are already dead, but the WHO says not to worry
Although it is the deadliest recorded outbreak of the rare Marburg disease, WHO spokeswoman Fadela Chaib said it can be controlled.
"Marburg is less severe than Ebola," she told reporters, saying an Ebola sufferer is capable of infecting about a dozen people but someone with Marburg infects only about four others.
I think (and hope) this is about right. One problem evolutionarily for viruses like Marburg is that they kill too quicky to spread widely. HIV is so devastating, and spread so widely, in part because it infects people for a long time before it kills them. With Marburg, you die so quickly you don't have a chance to infect that many others. Of course, Marburg is spread more easily than HIV which requires sharing of bodily fluids.
In any case, there have been a number of human outbreaks in the past, including the first recognized one which occurred in Western Europe (Marburg, Frankfurt and Belgrade) apparently spread by imported African Green Monkeys. Per this CDC site, the fatality rate is 25-35%, much below that reported for the current outbreak where >90% have died. Of course, without sophisticated serological testing, asymptomatic cases in Africa are probably often missed.
Personally, I'd worry more about the Avian flu.