South African law generally tracks English and American common law, including force majeure clauses and contract interpretation. It all really comes down to the wording of your contract. If there is no exclusion for disease outbreaks, and if it has become literally impossible to perform your work, you may very well have grounds for a claim. Also, depending on your contract's forum selection and choice of law clauses, South African law may not be what governs your dispute. Feel free to give me a call if you'd like to discuss the details.