VIANA, Angola (Reuters) -- Almost three decades of brutal civil war has left Angola a smouldering ruin, but health experts say the conflict may have kept a lid on the HIV/AIDS pandemic that has ravaged much of Africa.

Now the advent of peace may fan the spread of the disease.

And like a patient whose immune system has been weakened by the virus, postwar Angola may be in such a sickly state that it could easily succumb.

"The health system has been devastated by war. AIDS opens up an entirely new battleground that at present we're not well prepared to fight," said one health official in Luanda.

During the war, big areas were closed to the movement of civilians, preventing the rapid spread of the disease.

But an April 2002 peace accord between the government and its arch-foe, the UNITA rebel movement, has dramatically altered the situation and Angolans are on the move again.

The fear is that they are taking HIV with them.

The first and last nationwide figures on HIV prevalence in Angola are four years old and put it at around 5.5 percent -- a terrifying level giving Angola the 25th highest HIV rate in the world -- but one that is far less than other countries in the region where prevalence rates are between 20 and 30 percent. ILL-PREPARED

Those figures may be set to rocket and few countries may be more ill-prepared to deal with the crisis -- despite diamond and oil wealth.

Since the end of the war, millions of people have been crossing the country or returning to it.

Nearly one-third of Angolans have never heard of HIV and almost 70 percent of the population is under 24. Add the world's highest fertility rate and you have a combustible mix to ignite a nightmarish pandemic.

UNICEF says that 2003-4 could be a "window of opportunity" for Angola to avoid the disaster AIDS has caused in other southern African countries.

"These next 12 months are absolutely critical in Angola," said Melanie Luick, head of UNICEF Angola's AIDS program. "Youth in this country are filled with determination and hope and it is up to government and the international community to harness this in the name of social change."

But this is a society that is already hurting.

Most of its 13 million people live in abject poverty.

Critics say much of the country's oil wealth has been siphoned into the pockets of corrupt officials. The indifference of the ruling MPLA to the plight of the poor suggests it may not be moved to action by a growing AIDS crisis.

Malnutrition is rife with nearly two million dependent on food aid. And hungry, weak people are less able to withstand the effects of the disease than those who are well-fed and strong.

A shattered infrastructure would make any national rollout of anti-AIDS drugs -- if the government were ever to take such a step -- a massive task.

Sixty percent of Angola's hospitals were destroyed during the 27-year conflict. GLIMMER OF HOPE

There are some glimmers of hope on the horizon.

At the Jango Juvenile youth centre in the working-class suburb of Viana, on the outskirts of the capital Luanda, the UN children's agency is running a program to raise AIDS awareness.

In the playground a group of girls giggle at three boys rehearsing a play they have written on HIV and how to prevent it.

In another area of the playground, a girl appears from one of the classrooms with a condom braided into her hair.

"That is the sort of thing that demonstrates the youth centers are helping Angolans to become more open and to talk about sex," said UNICEF's Luick. "Although the condom isn't going to do a lot of good in her hair, it's her way of saying 'safe sex, or no sex'. And that's the message Angola needs to hear."

The UNICEF concept is proving a hit with the locals in Viana. Hundreds of young people queued to register for the youth center and the waiting list is growing every day. "These centers are not just about having HIV or not," said Luick. "They're about giving kids the ability to enter into the formal economy as well as to get a better education...and through that they have less of a chance of contracting HIV."