By Jennifer Rigby LONDON (Reuters) – The World Health Organization is working on further guidance for countries on how to mitigate the spread of monkeypox, amid concerns cases could spike further in the summer months, a senior adviser for the U.N. agency told Reuters. The WHO’s working theory based on the cases identified so far is that the outbreak is being driven by sexual contact, said David Heymann, chair of the WHO’s Strategic and Technical Advisory Group on Infectious Hazards with Pandemic and Epidemic Potential. He led a meeting on the outbreak on Friday. Monkeypox is an infectious dise…