Polio is caused by a virus, and symptoms of the disease can take between 3 and 21 days after infection to show.
The polio virus spreads between people through contact with infected faeces and throat secretions.
Poliovirus infection occurs in the gastrointestinal tract (stomach and gut). In 90 per cent of cases, the illness has no symptoms.
Where symptoms do occur they include:
headache; nausea and vomiting; tiredness; neck and back stiffness; and severe muscle pain.
Polio can cause meningitis (brain infection) and paralysis.
Both [polio] vaccines are cultured on animal tissue and are known to be contaminated with many monkey and other viruses. Only 3 of these - SV-40, SIV and Simian Foamy Virus, have been studied. SV-40 and Simian Foamy Virus are both associated with the development of cancer in humans and laboratory animals. SIV (Simian Immuodeficiency Virus) is thought to be the cause of HIV infections in humans, leading to AIDS infections.
An outbreak of polio in Congo-Brazzaville has killed more than 100 people and paralysed hundreds more, the authorities say.
Congo-Brazzaville had previously recorded its last case of indigenous polio in 2000.
The government said the vast majority of deaths had occurred in the coastal city of Pointe-Noire.
Polio damages the nervous system, causing paralysis or death if untreated.
Congo's director-general of health, Alexis Elira Dokekias, said the victims had either not been sufficiently immunized or not immunized at all.