Zimbabwe has one of the highest rates of HIV and AIDS in the world; nearly 15 pecent of the population carry the virus. Hundreds of thousands of Zimbabweans are kept alive by anti-retroviral drugs (ARVs) given freely by aid agencies on condition that they are not re-sold and that they are used for treating those in need. The drugs have played a vital role in stabilising a situation that was running out of control a decade ago, citing the death rate by more than two-thirds.

However, any interruption in the supply of those drugs to a patient can seriously affect their health, as HIV expert Professor Rudi Luthy of the Newlands clinic in Zimbabwe's capital Harare explained.

"Because HIV is stored away in many cells that are long living, the moment you stop the treatment these cells start replicating the virus - the virus grows and then the next dose comes, and then resistance emerges. You need to take your drugs religiously - you must not miss a single tablet .... if you interrupt or stop the treatment or take it every other day, it's bound to fail."

Which is why it is so disturbing that the regular supply of ARV is reportedly being disrupted by theft and corruption.

While most patients have experienced no problems maintaining their dosages, other have begun to notice shortages. Lengie Francisco is one such. She is 38 years old and has lived with HIV for nearly eight years. She picks up her ARV prescription from a local clinic every month but recently she has begun to notice that the sealed pill bottles have been opened and the number of tablets inside have been cut dramatically.

"Some people are saying is that there's a shortage," she explained. "Perhaps they're trying to share them out equally – but they don't want to tell us the truth. Sometimes when you go to the clinic they tell you 'wait, we haven't got any supply yet'."

The problem is even worse in rural areas. An HIV sufferer who lives in Mhondoro, five hours from the capital, explained that the drugs were becoming harder to find: "I went to the hospital and I could not get them. You can only find it on the black market."

As Zimbabwean health reporter Cassim John discovered while making this disturbing film for Africa Investigates, the medicines are somehow being siphoned out of hospitals, clinics and the national pharmaceutical network and then sold on the black market – often for use as recreational narcotics.

So who is responsible for this illegal trade and why is it being allowed to flourish?

Watch more Africa Investigates for reports by undercover African journalists who face intimidation, beatings and death threats.