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Filoviruses:

Actually, the heading should read Hemorrhagic viral fevers, but since the  filovirus group (a group of viruses which includes the deadly Ebola and Marburg viruses) are the most likely candidates for being converted into biological weapons, I decided to make the heading more specific. Filoviruses are so named because they are shaped like tiny hairs.

Hemorrhagic viral fevers are characterized by bleeding from wherever bleeding can occur. There are large patches of bleeding under the skin, bleeding from the gums, the nose, the mouth, the anus, in the urine, and from every other conceivable opening. There are also reports of bleeding occurring from the nipples of a man affected with the Marburg virus. The bleeding is rapid, profuse and unstoppable. The blood that flows out of the victim is unable to clot, forming a liquid pool around his body. I do not exaggerate when I say "pool of blood". So horrible is the disease and so infectious is the agent causing almost no one is ready to look after the victim who has to be kept segregated in the most extreme isolation.

Images: 
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1) Scanning EM picture of the Marburg virus. Notice the elongated shape of the virus.
2) Extensive bleeding under the skin (ecchymoses) in a person affected with Ebola.
3) Disinfenction of a dead body by international workers in Zaire in 1995. In the previous outbreak in 1976, the same hospital had to be shut down because Ebola started spreading among the hospital staff and the other patients of the hospital.

The victim generally dies in a few days ( around 1 week after the appearance of symptoms) of hypovolemia, or from liver failure and death in such a condition is possibly a relief.

The USSR was involved in making biological weapons with Marburg at their VEKTOR facility in Siberia. The Russians are also suspected of having made a "chimerical" organism by making a hybrid of Ebola and Smallpox (EbolaPox).

Image :  VEKTOR, Siberia.

Manufacturing Ease:

It is enormously difficult to grow these viruses in the laboratory. Sometimes, the only way to grow these viruses is to inject them into animals in whose bodies they replicate and increase in number. Doing even this is very difficult technically.
However, considering the fact that Marburg and Ebola figured significantly in the USSR weapons program, if is conceivable that a secret method for cultivating these organisms in large numbers exists.

Ease of delivery :

These viruses are extremely fragile. They need to be preserved in liquid nitrogen containers for being transported from one place to another e.g. from the source of an epidemic to the diagnostic laboratory.

0 Marks here too.

Infectivity: 

The infectivity varies for different members of the group. As far as Ebola is concerned, it is highly infective. (The infective dose may be as small as 1-10 viruses)
During an outbreak of Ebola in Zaire (in 1976), a single case of Ebola admitted to a 120bed hospital caused an outbreak of the disease among the patients and nurses that was so bad that the hospital had to be eventually shut down.

However, the viruses do not spread very easily through the air or by skin-to-skin contact. Hence, the potential of a disastrous outbreak is greatly reduced.

Mortality:

The mortality , too, is different for the different viruses.

1 out of every 4 people infected with Marburg die. However, the mortality of the Zaire strain of Ebola is a stupendous 90%!

Of course, Antibiotics are of no use against any of these diseases. Doctors just have to sit back and hope for the best.

Interesting Facts:

1) Only 2 facilities in the US ( CDC, Atlanta and USAMRIID ( previously Fort Detrick), Maryland) have the necessary safety facilities for handling such viruses.

2) The last known case of Marburg was in April 1999 . Where does the Marburg virus disappear in between outbreaks? There must be a place where the Marburg virus remains latent and hidden in between the periods when it is causes clinical disease in humans. 

Strange as it may seem, scientists have been able to pinpoint the location where it might be hidden. It is in a relatively small cave heavily infested with bats called The Kitum Cave on the slope of Mount Elgon, Kenya.

3) Before 1999, the  last known outbreak of Ebola had occurred April 1995 in Zaire. Around 240 people died.
The spread of the disease was halted because of the large degree of clinical suspicion regarding Ebola. CDC diagnosed the disease within hours of receiving the samples and a large international team with containment units poured into Zaire to restrict the spread of the disease.