As the Zika virus continues to spread throughout Latin America, leading health officials are calling for the 2016 Olympic Games in Rio de Janeiro to be moved, delayed, or both. The virus has affected over a million people in Brazil, and the number of babies born with microcephaly since the outbreak started is just under 5,000. The Olympics are scheduled to begin on August 5, and the massive influx of tourists could rapidly increase the global spread of the Zika virus.

Writing in the Harvard Public Health Review, Dr. Amir Attaran of the University of Ottawa has warned that holding the Olympics in the middle of an outbreak could ultimately result in a “full-blown global health disaster.” The nearly 500,000 expected tourists attending the event would be entering the very center of the outbreak in Brazil.

Since the Brazilian government began tracking cases of the Zika virus nationwide in January, Rio de Janeiro has emerged as the state with the most confirmed cases (26,000) and the fourth worst infection rate (157 per 100,000). The infection rate is expected to decline in the cooler August weather, as usually happens with similar diseases like Dengue fever. However, there are signs that it may still be spreading at elevated levels. For the first quarter of 2016, there has been a six-fold increase from a year ago in the number of Dengue cases within the city of Rio de Janeiro (8,133 cases compared to 1,285).

The Zika virus can also be sexually transmitted, allowing it to spread in far colder conditions than the 70°F winter lows of Rio de Janeiro.

Despite this, the International Olympic Committee (IOC) has decided to proceed with the games, claiming that there is “no justification for canceling, delaying, postponing or moving the Rio Games.’’ The World Health Organization (WHO) issued a set of recommendations for athletes and tourists but has taken no official position on delaying or moving the Olympics, with potentially far-reaching consequences.

An analysis published last March in Science shows that the entire Brazilian outbreak stemmed from just one infected traveler between May and December 2013. Globally, over 2 billion people live in regions where the weather and mosquito populations are conducive to the spread of Zika.

The long term impact of this strain of the Zika virus is still unknown. A new study by Brazilian researchers demonstrated that the strain in the current outbreak, independently of other factors like malnutrition, caused microcephaly in mice at a much higher rate than older strains. Aside from the sharp spike in congenital defects, Zika is also connected to an increase in rare neurological disorders like Guillain-Barré syndrome among adults infected.

Both microcephaly and the temporary paralysis associated with Guillain-Barré are immediately obvious conditions. Whether this strain of Zika can cause longer term complications for adults or children who do not develop microcephaly is simply not known. According to Dr. Muotri of UC San Diego, “Media covering the Zika story have focused upon affected babies with small heads because such images are profoundly dramatic, but the true health impact is likely to be more widespread and devastating.”

As of May 11, 58 countries have reported continuing mosquito-borne transmission, according to the WHO. Two cases of microcephaly have been reported in Slovenia and the United States, both due to a visit to Brazil.

The first Zika-related death in the United States was an elderly man in the territory of Puerto Rico. The man who died developed a low platelet count as a severe side-effect of infection with Zika, called immune thrombocytopenic purpura. Health officials in Puerto Rico said they tested over 6,000 people for Zika. Of those, 683 had the virus, 65 of them pregnant woman, 11 needed immediate hospitalization and five had Guillian-Barré, according to the CDC.

In Mexico, there have been 280 cases so far, most of them occurring in the southern states of Chiapas and Oaxaca, according to Mexico’s Health Secretary’s office.

There is a strong potential for transmission into the US, especially in the Southern states. Cities bordering Mexico are at higher risk due to crossings between the United States and Mexico. “It’s not a matter of if it’s going to happen, it’s a matter of when.” said Esmeralda Guajardo, Cameron County’s (Texas) health administrator. The Centers for Disease Control has reported 472 travel-associated cases of Zika since May 4.

There is a fundamental class basis for the outbreak of Zika, and the areas where it has spread rapidly have all been poverty stricken. Basic sanitation, window screens, mosquito nets, and running water would have been sufficient to prevent the outbreak of the virus. The epicenter of the outbreak was in Northeast Brazil, where 35 million people don’t have running water and over 100 million lack a sewage system. The poverty and overcrowding that allowed Zika to infect over a million Brazilians in just a few years exist in poverty stricken regions of the US as well.

In a statement released May 12, the WHO recommended that Olympic athletes and tourists “avoid visiting impoverished and over-crowded areas in cities and towns with no piped water and poor sanitation (ideal breeding grounds of mosquitoes) where the risk of being bitten is higher.”