NAIROBI (Reuters) - An outbreak of plague has killed 20 people in the space of a month in Madagascar, with a further 84 infected, the World Health Organisation (WHO) said on Friday. Plague is mainly spread by flea-carrying rats. Humans bitten by an infected flea usually develop a bubonic form of plague, which swells lymph nodes and can be treated with antibiotics.  But the more dangerous pneumonic form invades the lungs and can kill a person within 24 hours if not treated. About half of the 104 known cases are pneumonic, the WHO said. WHO spokesman Tarik Jasarevic told reporters in Geneva that areas affected included the capital Antananarivo and the port cities of Mahajenga and Toamasina. The U.N. health agency said it feared that the outbreak could worsen because the season for plague, which is endemic in Madagascar, had only just begun, and runs until April. On average, 400 cases are reported each year.  The overall risk of further spread at the national level is high,  WHO said in a statement. 