The sea wasp box jellyfish is perhaps the most deadly variety. This translucent sea-dweller may not look all that menacing, but it is the most venomous animal on planet Earth. Box jellyfish are deadly to many different animals, not the least of all, us. If you get stung by one of these animals, you are very likely to die.
Earth and Mars Conjunction: On the opposite end of the scale, Mars and Earth can be 401 million km apart (249 million miles) when they are in opposition and both are at aphelion. The average distance between the two is 225 million km. When Mars and Earth are at their closest, you have your launch window.
The problem is that there's an immense distance between Earth and Mars, which means any trip to the red planet will take a very long time. It's also made more complicated by the fact that the distance is constantly changing as the two planets rotate around the sun.