And now there’s a new layer to their rivalry: Musk surpassed Gates as the world’s second-richest person. His net worth now stands at US$128 billion.

Here’s how the friction between Gates and Musk began and everything that’s happened since.

Gates doesn’t appear to be a fan of Tesla vehicles, and he’s said as much publicly. In February, Gates said during an interview with YouTuber Marques Brownlee that while Tesla had helped to drive innovation and adoption of electric vehicles, he recently bought a Porsche Taycan.

Gates’ comments didn’t get by Musk, who tweeted that his conversations with Gates had always been “underwhelming”. But things really heated up in July when Gates took issue with Musk’s comments on the coronavirus pandemic.

In an interview on CNBC’s Squawk Box, Gates criticised Musk’s comments on the virus and implied that Musk shouldn’t be speaking about the pandemic at all.

“Elon’s positioning is to maintain a high level of outrageous comments,” Gates said. “He’s not much involved in vaccines. He makes a great electric car. And his rockets work well. So he’s allowed to say these things. I hope that he doesn’t confuse areas he’s not involved in too much.”

Gates and Musk are both involved in fighting the coronavirus. Gates pledged US$100 million to fight the virus via the Bill and Melinda Gates Foundation. Musk, meanwhile, redirected Tesla resources to source and produce ventilators and teamed up with the German biotech firm CureVac – in which Gates is an investor – to make a device to aid in vaccine production.

But since March, Musk has frequently downplayed the severity of the virus and strongly criticised stay-at-home orders. He’s promoted the malaria drug hydroxychloroquine as a treatment for the virus, falsely said that children are immune to it, and questioned data regarding coronavirus deaths.

Musk also said March that there would be “close to zero new cases” by the end of April. The death toll in the US alone recently surpassed 200,000.

Gates’ comments struck a nerve with South African-born Musk, who taunted his rival on Twitter. He jokingly posted multiple tweets about Gates, including “Billy G is not my lover” and “the rumour that Bill Gates & I are lovers is completely untrue.”

Musk has also tweeted that Gates had “no clue” about electric trucks. 

In August, Gates wrote a blog post about electric vehicles that questioned whether it was practical to make vehicles like 18-wheelers fully electric. Although he didn’t mention Tesla, the company does produce a semi.

A few weeks later, a Twitter user asked Musk about his opinion of Gates‘ comments, to which Musk replied, “he has no clue”.

On top of that, Musk recently called Gates a “knucklehead” for criticising his coronavirus efforts. During a podcast interview published this week, The New York Times asked Musk about his feelings about the virus, including whether he planned to get vaccinated.

Musk defended himself against Gates’ comments, saying he’d spent time with Harvard University epidemiologists “doing antibody studies” and noting Tesla’s work with CureVac.

“Gates said something about me not knowing what I was doing,” Musk said. “It’s like, ‘hey, knucklehead, we actually make the vaccine machines for CureVac, that company you’re invested in.’”

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This article originally appeared on Business Insider.