Lior,
Sure, but in order to stop a virus you would also have to stop people who had contact with anyone who may have traveled to any of those countries, and then anyone who may have had contact with those people etc.
For example: Tom from Liberia can travel to Mozambique where he has contact with Fred from Haiti. Fred now returns to Haiti and has contact with his cousin Carlos from Dominican Republic. Carlos’s brother Elvis works in Florida and comes to the Dominican Republic for the holidays where he spends the holidays with Carlos and then flies back to Florida on a plane with 250 passengers from all over the USA.
The USA can have the strictest laws blocking travel from Liberia. It won’t help.