The human body is adapted for living here on Earth and nowhere else in the universe. Just a few of the problems we’ll find going to Mars are:

The effects of weightlessness. Astronauts who have been on long-duration flights to the ISS have experienced vision problems, cardiovascular issues, bone loss, elevated CO2 levels, reduced cognition, and more.

All of these issues will be exacerbated on a Mars trip because the shortest trip to Mars would be far longer than the longest any human has ever been in space (Valeri Polyakov spent 14 months in space in 1995).

Radiation and cosmic rays will be an issue. Almost all of the time we’ve spent in space has been in low Earth orbit (LEO), which is under the Earth’s magnetic shield. The trip to Mars would expose the passengers to all the solar radiation and comic rays that our magnetic shield blocks, and studies have shown that the 24 Apollo astronauts that flew to the Moon showed a 5x greater incidence of cardiovascular disease later in life.

Immune system issues. Studies have shown that astronauts’ immune systems are reduced when weightless and bacteria become stronger at the same time. Outbreaks in the enclosed environment on Mars would be an issue.

The conditions of Mars. Mars only has 1% of the atmosphere of Earth and even that is carbon dioxide, which we can’t breathe. It also makes temperatures vary widely. Plus the soil is filled with perchlorate, which affects our endocrine system and can cause breathing problems.