Smallpox is a viral disease. It primarily affects the skin but it can affect many other systems. It is acute and highly fatal. It's initial symptoms are:
High fever
Head and body aches
Sometimes vomiting
By day four of the disease skin sores will fill with a thick, murky fluid and often have a divot in the middle of the sore.
While the person is contagious during the initial phases of the disease, they are most contagious once the sores are visible.
A rash will start as small red spots around the tongue and mouth. They will develop into sores that burst and put out large amounts of the virus into the mouth and throat
By day 6 of the rash the sores will become pustules. This stage will continue for around 10 days. Pustules are highly raised bumps on the skin that are round and firm like you have a bead under your skin. After the rash appears most of the sores will have scabbed over.
Once all of the scabs fall off then the person is no longer contagious.
But, It has been eradicated through the use of vaccines
In 1959 the WHO began an effort to eliminate all Smallpox from the earth. They intensified the effort in 1967 and through the use of vaccines they were able to eradicate the disease. The only Smallpox left in existence is in labs throughout the world.
This is a remarkable achievement as 3 out of every 10 people who were infected died of the disease.
Weaponized Smallpox
Because smallpox is an extraordinarily deadly disease, it has been a target for people looking to develop bioweapons. Terrorists and certain hostile nation states have been trying to get their hands on the virus and make it more virulent and deadly.
Because the smallpox vaccine has no longer been regularly administered, there is virtually no immunity built up. So, any attack is nearly certain to be disastrous. The first case will likely only be found once someone goes to the hospital exhibiting symptoms.
Treatment
There is no treatment for smallpox. The only thing a medical provider can do for you should you get sick is treat symptoms and prevent dehydration. If anything stops the disease it will be your own immune system.
Sources:
http://www.mayoclinic.org/diseases-conditions/smallpox/basics/treatment/con-20022769
https://www.cdc.gov/smallpox/index.html