THE spread of Ebola outside West Africa has raised fears that the deadly virus has become more easily transmissible.

Singapore, the United States and some European countries have recently responded to the threat by setting in place entry screening measures - including the taking of temperature and filling up of health questionnaire - at airports and rail terminals, after infected cases were confirmed in the United States and Spain.

Here's a look at whether the inbound screening works and how Ebola has evolved:

1. Does screening at airports and rail terminals work?

Singapore, France, Britain, the US, Canada and Czech Republic have recently announced new measures to screen travellers at certain airports and rail terminals.

But unlike in the later stages where symptoms like haemorrhagic fevers are easy to spot, the virus is hard to detect at the beginning stages. Typically, symptoms appear between eight and 10 days after exposure to the virus, but the incubation period can span two to 21 days.

The disease is not contagious before symptoms appear; but once they do, most patients will be too sick to travel.

But the screening is not without its merits.

Professor David Evans, a virologist at the University of Warwick, says that while testing passengers is "unlikely to detect symptomatic cases" as they arrive in this country, "the introduction of inbound passenger testing will both raise awareness and provide information that should ensure that passengers who subsequently develop symptoms can rapidly seek medical advice and, if needed, treatment", The Telegraph reported.

2. How has the virus mutated?

The Ebola virus has mutated repeatedly as it sweeps through West Africa, according to scientists tracking the genetic sequences of the virus in victims. The strain now circulating in West Africa's latest outbreak is different from the earlier strains. Researchers have identified more than 300 new viral mutations in the latest strain and are studying if this strain of the disease is more infectious.

It is feared that the longer the epidemic continues, the greater the chance that the virus could change in a way that makes it more transmissible between humans, making it harder to stop.

But evolutionary biologists have tried to debunk the belief, pointing out the lack of evidence to prove that new mutations in the Ebola virus are responsible for the huge size of the current outbreak.

3. How deadly or infectious it is?

The fatality rate in the current outbreak is about 70 per cent. The virus, which is not airborne, is only transmitted by contact with the bodily fluids of an infected person who has fever, diarrhoea or vomiting. Patients are most infectious at the late stages of the disease when large quantities of the virus are present inside the body, which is why healthcare workers and family members tending to gravely ill patients are especially at risk.

The Ebola virus is less transmissible from person to person compared to the Severe Acute Respiratory Syndrome (SARS) virus. SARS is transmitted more easily, often from an infected person's coughs and sneezes.

Typical symptoms of Ebola include high fever, severe headache, muscle pain, weakness, diarrhoea, vomiting, abdominal pain, unexplained haemorrhage (bleeding or bruising).

4. What is the worst-case scenario in West Africa?

Without additional intervention or changes in community behaviour, the Centres for Disease Control and Prevention (CDC) estimates there could be up to 1.4 million Ebola cases in West Africa by January 2015. The World Health Organisation has also provided a grim outlook, projecting between 5,000 and 10,000 new cases a week by the beginning of December.

The death toll so far in the outbreak, first reported in Guinea in March, has exceeded 4,000.

Sources: Bloomberg, AFP, CNN, Centres for Disease Control and Prevention (CDC), NYT, The Telegraph, Washington Post, Time magazine